Thanks for the replies. I sent an email off to a Colchester Camera Repair - a Canon repair center listed on the Canon website and got this response:
Thank you for your query.
To supply and fit a replacement friction ring only, will be £40 labour, £28
part, plus VAT and postage. To service, calibrate, clean, check and replace the
friction ring will be £130 labour, £28 part, plus VAT and postage.
If you would like to send your equipment to us, please include your details, a
description of the fault and a copy of this email.
It is advisable to have the parcel insured whilst in transit, which would be
Royal Mail special delivery or similar
If you follow this link
http://www.camera-repair.co.uk/wp-content/uploads/2011/04/Camera-Repair-Doc.pdf
you will find our packing label.
For your information, our current turn around time is roughly ten days
Kind regards,
The enquiries team
Think I'll give them a try unless anyone has any horror stories?!