there is one electricity company that charges the same rate for each electric unit wheather you pay by meter / direct debit.
but all the big ones charge a bit more for a unit of electricity when you pay on a meter compared to direct debit.
have you kept notes of your meter readings?
if not do that for a while (ideally a year) then you know exactly how much electricty you use (so dont guess, and dont have the companys guess 'as a 3 bed house, occupied 5pm - 9am and weekends, with insulation' etc)
then you can put your exact useage into energy comparison sites and see excatly how much that excat amount of electric would cost you...
if you use Xunits per year at Yp (of course you get X units and Yp and then the rest at Zp, so do that too ) you can know how much you should pay (also add any standing charge, and take away a direct debit money off bonus thing etc)and divide it by 12 for your approximtae monthly direct debit payment.
I did that (and it has worked out exactly correct)... alsoi did it on a site were they then gave me £30 into my bank just for switching
I think the problems come when its based on typical readings rather that actual readings