I think it is worth raising the complain as some would qualify this as a fraud attempt. I had the same issue with Hertz in Canada and raised the issue with the head office, explaing that I felt the numerous attempts to cheat me as being a fraud. Interestingly it was also on a pre-paid voucher.

After a few e-mail exchanges and a threat to complain to the authorities, they "investigated" the matter with the branch (or franchisee?) and decided to refund the whole amount as a commercial gesture for the way I have been treated.

Based on this experience, I would say it is worth explaining the situation. I would do that to Avis Germany but if you are a regular customer in the UK, it could also be the best place. I think the european headoffice for Avis is in the UK anyway.