As per the title really is it necessary to change the oil when a new cambelt is put into place?

Need to put mine in the garage to have the cambelt done, its already had a service 4k ago though - i've put the car on fixed servicing now too so didn't want them putting in different oil afterwards / during to top up or whatever.

Other than the cambelt and waterpump is it worth getting the auxilary belt done too at the same time? What price are you looking at at a steallers?

No, no need for an oil change. I would get the aux belt changed though, as it has to come off.