While I appreciate the significance of insulation etc, the point is
that the house is the same as it was when it was on oil. If anything,
it is now significantly better insulated, with the upgraded loft
insulation and cavity wall. So to (at least) double the cost of the
heating fuel seemed ridiculous. Unless there is something wrong with
my boiler I'm going to convert back to oil - Calor are due to ring
me...

Thanks for all the help and advice so far though - it's all appreciated.