Just putting this out there, others will have to confirm if this is viable or not...
As you tight on money at the moment, could you net get the guy to do it for £250, then if he gets it wrong take it to diamond motors to be timed correctly? I don't think you would need a new belt if it was done in a short space of time? That's way, you might get it done alright for £250 by luck, and if not you can get the timing corrected for another £150ish, bring you to the same price as what's normally charged?
dan etc will confirm if this is a really bad idea or not.