I think we can save you some money!
Cambelt. Get the DVD and do it yourself. Or bring it up to Nottingham sometime and I am sure someone here can fit it for a heck of a lot less than £300!!
Vibration could be wishbones also. How is the car going over potholes. Does the front end "crash" throught them with sharp vibration through the steering?
If it is discs, then Vauxhall are having a laugh!
Set of genuine discs and pads is just over £70 on trade club and they are a doddle to fit.
Finally tyres...and NO this is not an invitation for a tyre thread!!
Michelins will not get recommended here...
I have used Goodyear Eagle F1's and Continental Premicontacts. Both are fantastic tyres and are around £90 each fitted and balanced.
Mark has tried some Uniroyals recently and is very impressed with them (he can tell you the type).
Mytyres or black circles are both good places to look for tyres and a pair will cost considerably less than £280!