How much do these gas ones typically charge? Currently, i've tested it twice... it appears to charge roughly 1p per minute when the radiator's are on.. 20mins, 22[ charge, and couple of weeks ago, it has been on an hour and charged £1.20ish. This too high? I wouldn't of thought if the radiator's were on for 10 hours, it would charge anywhere in the region of upto £15. Seem's abit pricey..