Scottish Power/Manweb unfair apportioning

I don't know if anyone else has had the same problem as me with S.P or any other energy provider but it made me so annoyed I am writing this here.
Basically when my last gas bill arrived -- well over estimated as usual-- I noticed that there had been a price increase. The increase was at the start of October and the bill for the period 31 July to 6 Nov. I also noticed that SP had allocated 1/3 of the units used at the previous cheap rate and 2/3 at the new increased rate even though it would have been fairer to split it the other way round. After querying this I was told that because it was later in the year I would have used more gas then and that was the reason for the uneven split. I happened to be on holiday in October which made me even more determined to get them to split the units evenly. It took 3 days of trying to get my point across and I eventually quoted their complaints code which worked a treat. 5 mins later the split was even and I saved myself about £4.
The main point being I only have a small gas bill and was still better off after a more fair split. I wonder how much SP have managed to add on to the bills of people in similar circumstances and got away with it.
If this has been brought up before or is in the wrong thread, I'm sorry. I did have a quick sqint through the forums but did not see anything.