Part of the reason husband and wife team Stephen and Juliette Wall set up Pho was because Jules did not want to end up feeling the same way about Vietnamese fast casual dining as she does Cat Milk, they tell M&C Report.

The Durham-raised food science graduate insists she had had the idea and written down a proposal for the feline beverage before it ever hit the shelves. Mancunian Stephen, who met his wife 22 years ago at Sheffield Hallam University (a ‘poly’ when they started, they gleefully point out) when he was studying for the “absolutely rubbish” business and marketing degree he did as a slightly more mature student who had decided a career in management at H Samuel was not for him, laughs at the notion that Jules still feels aggrieved over this.

However, he says together they were determined that the idea for a restaurant based on fresh noodle soup, picked up from their eight months of travels after Stephen’s redundancy from a “dream job” with a sponsorship agency dealing with the McLaren Formula 1 team, would not go the same way.

Jules was also ready to take stock of her career at that time, having drifted into the world of marketing after jobs including “fascinating” lab work at the Boddingtons Brewery, which involved tasting halves of Stella in the morning, and “a disgusting job” at a sandwich fillings factory in Kent, overseeing hygiene.

“I didn’t like the feeling of having no control over my destiny so we decided to go away and have a think about what we wanted to do,” Stephen recalls. “We weren’t hung up on coming back with an idea, we just went to travel and jot ideas down. It seemed natural to go into food because of Jules’ background and because we both like it so much. Noodle soup was the first idea we wrote down because we liked it and we were big customers of Wagamama back home and loved our local Vietnamese in Hackney, but realised there was a gap in the market. We knew we would be really annoyed if someone else did it.”

Ceviche, the popular South American seafood dish, was another of the ideas, noted down while they were in Peru – someone else has now developed a concept around it in Soho - but they have no qualms about the decision they took. It is now eight years after the opening of their first Pho, in Clerkenwell (which spookily opened a year to the day after they returned from their travels). The Walls are still just as devoted to Vietnamese cuisine and are all set for the next stage of the now circa £10m turnover, eight-strong chain’s growth.

In fact they seem remarkably relaxed as they discuss their plans in the week before the opening of the eighth site, in the fast-casual-dining showcase that is Upper Street, Islington. Even more remarkable given there is a broken wok, a chunk knocked out of the wall, plus deliveries galore and a visit by their landlord, all happening at the same time.

“It feels exciting doing an opening now,” Jules says. “Yes it is long hours and very busy, but we are more confident in what we are doing now.”

With a six-year-old son to look after, too, time is a commodity they are looking forward to having more of now that they have beefed up the head office team, ready to roll out to 20 to 25 sites in the next four to five years. The operations and finance teams are now in place and at the time of this interview the company was about to fill two more strategic roles, in marketing and site and project management.

The Walls are particularly happy that they also now have ex-Wagamama boss Steve Hill as Pho chairman: “We have never done what we are about to do and hopefully he will tell us what not to do,” Stephen says. “One of the key things he will do is make sure the management is sustainable, so Jules is not on a computer until midnight every night.”

Things have certainly improved from the first 15 months of Pho’s life, when both Stephen and Jules worked every shift between them. At the first restaurant, the chef handed in his notice on the second day because he found it too stressful. Looking back they both say they should have promoted others within the organisation sooner so they could delegate more.

Having said that, there have been benefits to the all-consuming nature of those early days – Jules reckons the fact that their now 200-strong workforce knows they have lived and breathed the brand in this way filters down and engenders a real loyalty and passion for the business among them.

Efficient central management will become even more pertinent once Pho opens a site in Leeds in October, its second site outside London after the phenomenally successful Brighton operation next to Jamie’s Italian (Stephen gets a real kick out of watching the buzz inside and the queues outside that restaurant and still can’t quite believe they made it happen). Luckily, or rather demonstrating how their passion for the business has filtered down to others in the organisation, they already have a head chef and a manager from within the company who are prepared to lead the northern charge.

Another site, this time in West London, is also on the cards for the latter part of this year, with a couple more properties in the pipeline for potential openings in early 2014 and at the end of next year.

Aside from the odd too good to miss opportunities like Brighton, Pho restaurants cost circa £300,000 to £400,000 to get up and running. The new 90-seat Islington site is a model they are particularly happy with – rent and capex at a level that could realistically lead to ROI up to 40% within three years, based on average spend per head of around £12.50. However, Stephen is keen to stress that the Walls’ philosophy is to run the company focussed on customer experience, not profit and loss: “That way it will translate to the bottom line anyway,” he says.

Funding for the next stage of the company’s development is in place thanks to last years £5.2m staged agreement with ISIS Equity Partners, a set up the Walls are very happy with, thankful that they are not too reliant on the banks and that there is no pressure to take bad sites to fill unrealistic expansion criteria.

On their to do list is further brand and food development. They both think it wholly unacceptable that the size of a chain is too often in their eyes inversely proportionate to the quality of the food served. They are determined that will not be the case with Pho.

“When we change our recipes we usually add ingredients in to improve the taste, not take things out to save cost,” Jules says.

Every 18 months or so they go back to Vietnam to ensure the authenticity of their dishes stays up to scratch and to get ideas for new menu items. The last fact-finding tour earlier this year led to the addition of four Vietnamese spirit-inspired cocktails to the Pho menu (which of course, they had to test thoroughly themselves before deciding which ones to go with. They also ate a cricket salad, but funnily enough that did not make it onto the Pho menu.

The next busman’s holiday will be to Orange County, California, which has a massive Vietnamese community, to see how restaurants there adapt for the American fast-casual dining market.

It sounds fun, but even that fun is still related to the business and they know they need to work harder to get that elusive work/life balance right. As a partnership, they seem totally complementary, each concentrating on different parts of the business - Jules more on the kitchen and food and Stephen the finance and property. They do cross over on marketing and branding but amazingly, and totally believably given the relaxed easy banter between them, they rarely disagree.

Can they believe that their idea has grown to this credible, growing, recognised and well-backed chain?  “I never thought it would get to this,” Jules says, shaking her head. Stephen agrees.