Gareth,
I'd be careful on this one. Promises of future work are often used to squeeze prices. The chances are he just wants them all done on the cheap and you won't hear from him again, or they will be nightmare jobs in the first place.
I would give him a price based on bulk. In other words, if you give me three apartments on the same day £xx each and so on so that the price incentive encourages more jobs and if there isn't the bulk work he lured you in with, at least you're not doing them all for next to nothing, which is really what he's looking for.
Simon