It depends on a few things! Where you're located, how you pay, your usage (mainly) and scarily, the previous 'tenants'. When we were querying our electricity bills when we first moved in, they mentioned to us we could be on a rate that 'makes up' for the previous tenant not ever paying their bills - we were livid!! Plus, there's no such thing as an 'average' rate (which is what you are charged per 'kWh'), so you probably won't be paying the same rate as your neighbour with the same supplier, for example (its on your bill).
Try the utility switching search engines, as it will vary. There shouldn't be any hassle with the reputable companies as they'll do it all for you. Don't forget they'll earn a commission from the supplier so they bloomin well should do!!
For the record though, we're on Powergen!!
JulesP it sounds like you're gasman was a salesman for Southern Electric!! ;)