All UK electricity companies I've ever used are dodgy with their billing. They have _always_ over-estimated my usage which they always use as an excuse to over-charge me in advance. They constantly put up my direct debits to get increasing interest-free loans from me. They almost always use the excuse that energy use can vary across a year (obvs) so they need to build up a large stockpile of credit. That excuse sounds most plausible at this time of the year but I've had it in spring too.