Robin Knox dropped out of medical school at 19 to become a pub doorman, but eight years later he is advising RBS on an app for would-be successful entrepreneurs.

His third start-up Intelligent Point of Sale began building the UK's first iPad-based till system in 2013, took on its first employee last April, and by the year end was employing 14 and supporting 500 bars, hotels and retailers.

Its downloadable app, built in an Edinburgh kitchen, and £39 a month fee undercuts traditional £4500 electronic tills, competes with software from big US providers, and has won over customers such as Underbelly the UK's biggest festival operator.

The business is confident of raising £600,000 from the angel sector this year and is developing licensing plans which will see the business model replicated across Europe and beyond.

Its co-founder says: "With increasing amounts of SMEs looking to cut overhead costs and cater to increasingly tech-savvy customer bases, more are turning to innovative point of sale solutions which increase their control and efficiency."

Robin Knox's business may still be early-stage, but he is already seen as a model young wealth creator. "I am helping RBS make an app that tells you how to set up a business, I wish I would have had a tool like that, most of my knowledge about simple things in business has come from hours of googling. An hour's chat with someone who has been there and done it is worth a week's googling."

During secondary schooling in Aberdeen, where his family had moved from Buckinghamshire, and the short university career in Edinburgh, jobs in retail and hospitality piqued his interest. "I ended up working on the doors of pubs in Edinburgh, and when I went back to Aberdeen I did that full-time until I reached the point of realising I needed to get a life, get a job.....so I worked my way into night-club management."

Mr Knox applied to Luminar. "They're the biggest night club operator in the UK, it's an obvious training-ground and I knew I could work my way to the top. They didn't have any jobs but they gave me one anyway because I said I would move to Edinburgh to work for them."

His first boss at the capital's Cavendish club was industry veteran Bill MacGregor who had run some of London's top venues and trained the company's chief executive. "He was incredible to work for...in six months I was assistant manager then operations manager."

Then after a spell with a smaller operator, Mr Knox decided to use a sizeable lump sum, earmarked by his parents for a house deposit, on setting up his first business, after spotting that although paintball was popular its venues were usually hard to get to. "I wondered why nobody was doing it in a warehouse, and thought I'll do it. I rented two warehouses which had been vacant for years, and wanted to start advertising so I looked into hiring a billboard van. I quickly realised it would cost as much to hire as to build one, so I started up a second business advanscotland.com which paid for itself very quickly."

Urban Paintball Edinburgh prospered, helped by a £5000 loan from the Prince's Trust's Youth Business Scotland, and it prompted the need for proper revenue management. "I knew I needed an electronic point of sale system, I had used these systems since I was 13. But I had looked into renting a pub in the Cowgate and been quoted a £9000 set-up fee for two tills, that's a typical cost from the big boys."

The cheaper alternative is to run software on a PC linked to a touch-screen monitor, the entrepreneur explains. "Just the cost of the monitor was the same as the cost of an iPad. That was when the light bulb went on. You could have a computer with touch-screen, the best around, for the same price as a monitor."

Mr Knox approached former night-club colleague and technology manager Paul Walton, also 27, and they began to build an app, trialling it with a friend who was about to open a takeaway business in the capital. "I tried to convince Paul to leave his job, in January 2013 he did and we moved into a flat and worked out of the kitchen."

The next 15 months brought steady progress, £7000 from the RBS/Hunter Foundation Edge Fund, and a £25,000 loan from the Prince's Trust Growth Fund, which said IPOS had created "an affordable POS system for small businesses that was easy to use and self-install". The loan enabled investment in staff and equipment to upgrade the app, improving the user experience and freeing up Mr Knox for the marketing drive which saw a 200per cent rise in the customer base in 2014.

He says: "We thought we would run it all ourselves and that would be fantastic, but what has happened is that in order to keep up with demand from your customers - who will be screaming for what they want next - you need support staff and we have had to expand the company in line with subscription income. All the money from the business goes straight out in staff."

The snowballing roster of unique SME clients leaves IPOS less exposed to a large client walking away and "leaving me to make a team of 20 redundant", Mr Knox says. He expects headcount to grow quickly to 23 then plateau, with a core staffing platform enabling turnover to build to £3m over three years.

Now he is taking the model overseas, aiming to sign up electronic point of sale resellers as franchisees with a stake in success. "We don't want them to just sell and run. Traditional EPOS is sold on a licensing basis, and our competition operates on that model, but everything is moving to software as a service."

Mr Knox is quick to cite his own mentors Julie Greave (Abbey Business Centres) Bill Douglas (RBS mobile banking) and Stuart Harper (Electrum Multimedia) and IPOS has already joined the Edinburgh Guarantee scheme offering internships to unemployed school-leavers.

He is confident that his pitch to angel syndicates for at least £500,000 this year will attract interest. "It is not a concept or idea, we have a proven business to hand over and show them - software as service is secure and investable."