Thanks folks! Woke up this morning feeling a bit less glum. Yes it was bought from a dealer in Southampton, I live in Aberdeen so using a local garage to assess the issues and put them right. They are run by an ex-Mercedes master tech, we used them for our old M class a few years ago and they are first class. If they're telling me they don't believe the cam cover is cracked/just needed tightened I wouldn't hesitate to believe them, but they may just be mistaken - we'll see after a few more drives.
So far the dealer has authorised everything to be sorted other than the front wheels. The garage said they are fairly sure they wouldn't pass an MOT, although the car was MOT'd just before sale (possibly the usual pre-sale test done by Stevie Wonder) so I'm not sure what'll happen there. Hopefully they can be straightened and repaired properly, there is a good wheel place near us so I'll ask the dealer about that.
I believe to reject a car you have to give the dealer a fair chance to sort things out first. Not sure if they'd have the right to insist on having the car inspected themselves in person or at a garage of their choosing though which would mean taking it back to Southampton. If it really does come to that then that's what I'll have to do but we're not there yet. The only thing that I think might drag on is the wheels.