I think in the Circumstances, The Offer of Paying for All the Parts and a Labour Element of £250.00 is a very Reasonable offer, At Least they haven't Denied all responsibility, and Refused to pay anything. ?
I have always Been very wary about doing these Type of Jobs because of this Possibility of serious damage occurring, But If We are going to Tackle the Jobs, ( And make a good profit on them ) Then we should also be prepared to Accept a certain amount of the Responsibility when something goes wrong. ( Even though it's annoying Expensive and Embarrassing )

I would guess that they have covered your Cost's fairly well with £250.00 Towards Labour and In Circumstances like this, I think you should be prepared to work with the Manufacturer to Sort out an Amicable agreement that doesn't cost Either of you an Arm and a Leg. ( After all, You would have made a lot more out of the Job than they did ). If Repetative Failures Started to happen and it turns out that the Belts are Inferior, Then That would be a different matter. ?

( Also Add £20.00 on to Every Quote for a Cambelt change To Put Aside for Warranty Purposes, That way when anything goes wrong, It won't feel half as bad ).