Hi Gasman, it was me who advised you to take this from the law pages to the motoring pages.
I said I'd ask my brother-in-law (until recently an AA patrolman with 18 years experience). He has confirmed much of what has been written on here by the mechanics.
In a timing belt service you (should) replace the belt, tensioners, and in some cases (depending on how the belt is driven), the water pump. As I understand it, it is these other components that comprise the 'kit'. My B.I.L agrees with others on here that it is best practice to replace the belt & kit together., and that it would be best to refuse to do the work if asked to fit the belt only. Furthermore, he says many repairers actually follow this procedure. He also says he has seen scores of new timing belt slips/failures fitted without the new kit being fitted.
All in all, he feels the garage have made a more than reasonable offer. Fair enough - if the garage did what they were asked to!
On the legal side, I still advise (much like Mr T) to test the information the garage have given you. What do trading standards think - was it a 'new' timing belt? Was it ever fitted? If so, was it defective, or was the problem due to the old kit?
If the garage didn't do the work they agreed to do they are in breach of contract. The remedy is damages -- actual + consequential. If they did what was asked, take the offer and put it down to experience.
Best of luck!