The Mountain Boot Company will handle this I think... at least that's where the Grivel Website points you to on the contact tab: http://www.grivelgb.co.uk/contact/
Might be worth filling the form in and seeing if they can help?
In reply to tallpaulselfridge: yeah Mountain Boot Company are the people to contact. they're usually pretty good with stuff like that. I had an issue with the headtorch elastic on the back of mine and they sorted it no problem.