the previous comment was indeed correct you always need to check your meter readings when receiving your bills, having worked in the industry I can tell you there are many discrepancies in monthly payment some paying too much some too little the energy companies only monitor payments periodically it is up to you to ensure your meter readings are accurate, if your bills have been underestimated they will eventually catch up resulting in an accurate bill higher than expected and your provider putting up your payment to cover the previous under payments.
I would also say that to compare what you pay to a friend or relative can be very misleading, ie the previous commented his was far lower than yours , mine own is higher 2 beds 2 people £app £100 - £110 per month Averages mean little also All properties vary for so many reasons, insulation, type of boiler, age of boiler, also a boiler serviced regularly runs much more efficiently so cheaper, habits like keeping doors closed etc etc and also you as a person, I feel my house is comfortable other people find it too warm so I can understand why others have lower bills than me because they dont heat to the same degree.
I do think that electricity is harder to monitor and we feel we get better value from gas thats because it does 2 major things, heating and water but we rarely realise the amount of electric we use on so many modern gadgets.
In respect of a pre pay meter I think you may find that a charge to change over would be made as they are not so favoured by the utility companies other than for people unable to pay by normal methods.
you can have meters checked to ensure they are correct but believe you me this can be VERY difficult to get done and in reality it is extremley rare that meters are faulty.
Also you said you had just changed suplliers it may be worth mentioning that on change over the readings used are normally estimated also they are not given by the old co to the new but are estimates by the new company which can be inaccurate but this balaces out as it means your final bill to your last company may have been lower than it should have been resulting in your first bill to the new company being higher (and visa versa) there is only a problem if your final readings do not match to your start readings with the new company