lulumama I used to work for Npower, the prepayment meters do have a higher tariff as they cost more because of all the back end service, stay with me on this.
Every time you take your prepayment card into a shop and they put it into the machine to top it up, it sends you meter status to Npower or whoever, down a phoneline, it tells us EVERYTHING about your meter -
Meter readings, if you are in credit/debit, if you are tampering with the meter (seriously it sends us an SOS) etc etc,
Then everytime you put your card into your meter to transfer the money over it takes the meter reading and all other settings.
It means (and it is very big brother) we could always see when you topped up, where, how much for, what time of day/night you did it!
Personally I wouldn't go down the prepayment meter route unless you HAVE to. The topping up can be a nightmare, phonelines go down (recall hysterical call from bride on her wedding day where they couldn't top up the meter because the system was down, she had wet hair and needed to dry it) some areas don't have an all night garage or other late night place to top up.
The boiler will make a HUGE difference, it did to us, we had a 15 year old one that ate gas, had combi condenser boiler fitted, is a dream.
Stay on the fixed rate as in the long run you should do well. I don't know if it your gas or elec that is costing loads, my washing machine uses elec even when it isn't on so I switch it off at the switch.
Those elec usage things are about £30??? We are getting one.
Hope this long ramble has helped