Prepayment meters are a lot more expensive per unit than a standard meter anyway and utilities prices have shot up in the last couple of months anyway. All the providers have put prices up by around 15% in that time.
Winter is obviously going to be a bit more anyway - it's hard to say if you're paying a normal price without knowing if you have the heating on all the time, a well-insulated property, a bath or a shower and how any times it gets used in a day, how many people in the house etc etc.
For a comparison, I'm in a 2 bed semi detached and we use gas for heating, cooking and water. The heating is not on all the time, but we don't have a shower so baths are the order of the day. Our combined electric and gas bill was �270 for the last quarter so we're paying a fair bit less than you are.
Hope that helps a bit - if it does seem too expensive you should see about getting a normal meter fitted if it's an option. Cheaper and you can still pay in the same way - weekly, so no large bills