2000 Vauxhall Tigra oil burning after CAM belt change

I sadly neglected to have the cam belt on my engine changed after the recommended 40000 miles.
It broke, the top part was badly damaged, and the resultant work cost me over £900 (uk pounds).

Since receiving it back, the engine has been burning a lot of oil, which it did not previously do.
The mechanic concerned says that I must pay him for the parts required to put this right, (the pistons?). He will stand the cost of the large amount of work that will be required to do this.

Other people are telling me that it must be the valve guides which are in need of replacement, which the mechanic ought to have seen when he did the original job. Therefore I should pay no further mnoney to have the condition put right
Any advice please?