Unfortunately some letting agents charge whatever fees they like but usually the Landlord is responsible for renewal charges, not the tenant. Of course, it depends what the terms of your tenancy are, & whether the agent is responsible for management/upkeep of the property, or if they just find & vet the tenant for the Landlord.
Do you deal only through the letting agent or do you have contact with your Landlord?
If you do, I would have a word with him/her about this. If you have been a good tenant & always paid on time, why don't you just arrange to pay monthly in advance at the higher rate? Your references have already been checked so I would have thought that a current payslip showing your rise in salary would be sufficient.
Presumably you paid a Deposit when you first took out the tenancy, so this will be held in a special account in case of any problems.
I am a Landlord, & use an agent to find the tenant, but I like to do my own management & have a good relationship with the person renting my property.
Good luck.