Tooj, if it's of any help, we've just had a green deal inspector round to assess us for external insulation and his report showed that the average 3 bedroom house (concrete built terrace) with an occupancy of 2 was £1,600 per annum - based on the heating being on 11hrs a day.
If you took this for a brick built semi (holds more heat than concrete) - then I would guess that the £1,200 figure would show an average use if the heating was on for 12hrs a day.
(We don't have the heating on for 11hrs, so why the comparison was used I'm unsure (we fell way below the average use/ cost per annum).
Hope this helps a bit as to the mystery.