AndyB - good luck with your venture. I see this as a business with great potential, especially in towns where there are a lot of cycle commuters.
My impression of bike shops (and I'm prepared to be corrected by people who know a lot more than me on the subject) is that the servicing department is pretty profitable, and often lacking in capacity. For example, in London & suburbs, I frequently hear of a week and longer wait to book a bike in for a service (incidental evidence, I know).
So why not have a repairs specialist? Collect the bike from home / work. Fix. Return.
When I heard that Evans charge £15 + parts to change a tube (and seemingly, only if you rant that you *must have* your Brompton TODAY) I considered a mobile puncture repair service. For £10 + bits I would ride to your location in zone 1 or 2 and repair your tyre.