Hi Dave,
Bad luck ! You can try and swerve the responsibility but unless you got the customer to sign a "disclaimer" then I think it's gonna be down to your garage to sort it.
Officially you are not allowed to ask the customer to sign a "disclaimer" to the effect that if the engine blows up whilst emission testing then its their fault and not yours !! BUT you can ask them to sign a form that states -

I the undersigned confirm that the vehicle with Reg No........... has been serviced in accordance with the manufacturers specifications and that the timing belt has been inspected/ replaced (please indicate)

Signed .......... Date.......

If then something was to happen, and you could prove that it was due to incorrect service/maintenance then you have a "get out of jail free" card to show the judge !

Regards
Dave