G'day
I recommend that you phone the ProCentre in Mallow Street (near Old Street). They rent out loads of Hasselblad gear (mostly H system now). They either service their own stuff or get it done somewhere pretty good I reckon. They were always pretty helpful, so they may give you a steer. They used to have really good film prices too!
Good luck
Ian
I use Luton Camera Repairs - but they are a long way from London (roughly 25 miles north). The web site is lutoncameras.co.uk it is possible they will do a postal service maybe.