Try this place:
http://www.treasured-memories.org.uk/index.html
I've not used them myself, but someone I know who has used them was very pleased. They have all the right sort of equipment etc
(Nikon Coolscan 9000ED, probably the best film scanner ever) and seem to have good reviews from others on the internet as well.
They normally charge £31.00 for the first 50 negs/slides/prints and £0.40 per additional image. However there is a 60p extra charge for medium format (they take longer to scan, edit etc) so thats about a £1 per image which is quite reasonable for medium format. If you email them though they may be able to do a discount deal for you.
I would like to point out though, there are a lot of 'cheap' services out there which sadly reflect on their quality a lot of the time. I used one such service about a year ago (scanitall.co.uk) which was about 25p per 35mm neg/slide (they also did medium format for about 80p) so I though great and had them scan in about 120 negs/slides. When I got it back I was initially pleased and then realised that most of the scans were so soft and noisy (it was chrome noise, so certainly not grain) that the small lab CD scans I had of some of them looked better. A lot of them were lacking in saturation etc as well and whilst I could have potentially corrected everyone it wasn't worth it.