Thanks Sewgorgeous - it wouldn't really work because when I'm working at home I'm also in 'relaxed' mode (the bra gets undone as well and I have to do it up quick if someone comes round). And 'dressing up' to go dog walking seems a bit silly!
It's partly a psychological thing - if we have people round I can sit on the same seat in 'proper' clothes, bra done up, and be perfectly comfortable. Then the moment they go, I have to go into comfort mode again.
I should also investigate making low-rise leggings - after all, my knickers are comfy, which is just as well really!