its the law of diminishing returns.......
the people who did ours tried to get me interested in external wall insulation.. with a grant, it was £5k.... so i said no..
but, my missus has asthma, and we own a dog, so the heating is off in the day & the door gets opened.....
our biggest saving is that we don't have a power shower. or any shower.. we have baths... guess what?? we burn less energy to run a bath than to have a 5 minute shower... some people in the energy supply trade have been lying for years