Yes, we have cavity wall insulation, but not on the front wall of the house/living room as it has a tiled front. We have considered getting it covered/insulated, but it would be as much from a cosmetic POV as for insulation.
I think having decent double glazing, plus the fact I've doubled up on blinds and curtains in the living room (for home cinema purposes to get it as dark as possible) really helps retain the heat.
It also worked well during the heat wave to stop the heat getting in by closing the blinds/curtains and we left the living room door open so that cooler air would come in from the kitchen side (north facing) of the house. We managed fine without air con on those extremely hot days.
Regarding my 'jumping' unit prices: Someone posted a link on another website which has the correct rates for all areas on Octopus, so I've been able to work out my budget. A pleasant surprise because it's about £20 a month less than I'm paying (before the £67 reduction). I will leave the payments as they are so we should in theory help build a little buffer along with the existing credit we have on the account.
I'm now paying 34.23p a unit Electricity and standing charge is 42.77p a day. Gas is 10.46p a unit and standing charge is 26.84p a day.
Here is the link for anyone else with Octopus:
Energy prices from October Octopus