British Gas DD increase - accurate?

Brie

You need to calculate what your annual use is and then divide by 12 to get the monthly fixed DD.  Don't forget to use the current tariff you are on and that you also need to add the 5% tax.

My provider didn't do the calculation properly.  They took the amount of gas & electric we used last Jan/Feb and used that to figure out what our annual use would be.  Given that this was the highest use period in 12 months their estimate was ridiculous and suggested our DD increase from £120 to about £290.  When I contacted them they admitted that not only did they use a short period in winter that they had then added an extra 2 months worth to give the annual period 14 months which they then divided by 12.  I quoted back to them actual use, mentioned that due to personal circumstances that our use is decreasing rather than increasing and so they have agreed to reduce the DD to £80 a month.