Hi Sony,

As I mentioned in the previous answer, the rise in fuel consumption could be due to something done incorrectly during the timing belt change, but of course it might be unrelated.

Ask the garage if they'll check the tensioner and the timing itself. If they won't, I suggest you get another garage to do it, document and photograph the work and keep all receipts in case you need to go back to the original garage to get reimbursement.