Question Author
Tuvok, glad that system works for you, it wouldn't work for me as many energy companies charge different rates for the first x amount of units, the second x amount etc and some charge the same for all the units used. I haven't the time nor the inclination to do the maths for all the available suppliers.
Symbio only supply electricity and it usually (not always) cheaper for me to get both gas and electricity from the same supplier. That's another factor I'd have to take in to account if I didn't use the comparison sites.