I'd be a bit concerned too..
If you're not happy with the repair then you may have problems getting it resolved. The guy might just not agree that something needs to be done again or done properly (for instance. let's say you have a small dent on the nose.. You want it fixed, the guy says it was already like that. Who makes the final decision?).
You'll have a job going through his insurers later if you accept that he has offered to repair the car for you in the meantime. They'll say that they're only liable to repair your damage caused by the accident.. but meanwhile you've essentially already had it repaired... so it becomes a bit of a guessing game what they're actually liable for.
On the other hand, the fella could be genuine. If he fixes the car for free, he just has to pay a few parts and one of his lads can supply labour for "free". It could save him quids in his trade insurance (which I imagine is a bit tasty).
Get a written schedule of work so that you both know what is included and what isn't. Also get a guarantee on the work (whatever is the going rate .. 2 years?) The guy isn't necessarily out to rip you off.. but it saves arguments later.
If the bodyshop is Approved then I doubt they'll bodge anything... those guys see bodges all the time and really don't like it... but even so, get someone knowledgable to give it a once over afterwards (RAC or a friendly mechanic) in case they stuck the wheels on with masking tape or something.. you never know.
Ralf S.