To be fair, he could have cost you a nasty penny or two.
Due to the AA man trying to turn it over while belt BROKEN, I think the best course of action would be to get someone to take your head off and check your engine out before going any further.
If you assume no damage has been done and start playing with replacing the belt things could just get worse.
Considering the mechanic didn't notice the belt was broken, I would ring up the AA and ask them what they feel the chances are that his actions could have made the situation worse. You might be able to claim as cranking the engines without the cam shafts turning could be a really serious issue, and if it was the AA that caused it you should push to have them back repairing it.
OR it could be fine. But I would definitely want someone to check my engine out first, just because you say he has tried to turn it over with the valves stationary.
Trying to understand whats happening to your engine, if the belt is broken, the shafts won't be turning, therefore the valves would be in the same position regardless of the crack position. But if the pistons rise with the valves still in, there is a chance of damage?
Either way contact the AA and tell them the situation and ask for their advice. If there is a chance the engine was damaged before the AA man even turned up, they argue the fact he mad it worse.
This is just my opinion though mate, Im not a qualified mechanic, maybe someone else will have a better insight into the situation :-) Keep us informed!