Psy, totally my opinion as im a complete novice.we all sound in agreement that youve sadly been shafted here & as much as that really sucks, the forums expertise is more biased towards individual advice about specific items & you dont seem to be getting much luck.actually Bristol seems a bit strangely thin on the ground with good mechanics if this post is anything to go by.
It may make hard reading but being a regular visitor here, perfectly decent, good running fazers are going for £1000 & even less at times.
Your bike clearly needs an unknown amount of money thrown at it & who knows what damage your earlier mechanic has done.yes he should be making things right but obviously cant be trusted so can see why you just want clear of him.thats your choice but limits the options.
To hazard a guess id say youll be a minimum of £500 straight off & if no hitches.
Could it be time to cut your losses.
My idea - buy a complete second hand engine & gearbox, wait for one that is guaranteed to work.
Better still - buy a cheap fully running fazer for £800 to £1000 & use your bike for any spares needed to get it to the same condition & selling any unwanted parts to make some tidy money back.you'll be surprised what money you could get back from good condition front fairing, fuel tank,plastics etc.forget the engine but sell the gearbox & carbs etc.
Good luck!