Thank you -!- for all the responses and the personal stories, they really do help, honestly.
The house was valued at £600,000 in 2017 but the STBX dragged his feet, and the property that backs onto mine was extended (the owner died and sons inherited) .. and then they built a house within their garden very close to the boundary line with mine. I've spent £3,000+ probably on on specialist advice and solicitors against the planning permission being granted - granted anyway, property built not as the approved plans, being too close and too high; the Council put an enforcement order on it just as lockdown struck, but was deemed by the enforcement people to be allowed in August this year. Very disappointing. And the valuations I've had last month are now £550,000 to £560,000. Incidentally both houses are now for sale, the extended house was rented out for a while but neighbours have seen bailiffs so he's obviously had problems. The extended is a 4 bed up for £570,000, the new build in the garden (!) is a 2 bed up for £499,000. They look ridiculous, on the same plot of land, but it does give the impression of a glut of properties up for sale in the local area.
Paying off the mortgage of £55,000 and then the split of funds, with a good wodge of luck I'll get maybe £250,000. We purchased the house at £132,500 in 1999 with a gift of £20,000 deposit from my parents and my savings.
I've looked at 1 bed houses with direct links to London Bridge (my office is a 5 minute walk from there). I'm really quite anxious about the commute and having to stand for any length of time. And right now property prices are, I think, going to be wildly different to what they may be in 4 years time when I will want to move to a different area, I don't want to find myself with less now ...
Somerset was one of the areas I was considering, cheaper than Devon, less remote than Cornwall (DS intends to remain in London). I'm not really close to either of my sisters, one is in East Sussex = out of my price range, the other is in the process of moving out of London herself, near Aylesbury. Since the split with the STBX I've made friends on Meetup, which has been a lot of fun, but I've no ties really to anywhere! Stick a pin in a map .. And I'd worked out that budget wise it would probably be £11,000 to £12,000 to cover things like council tax, utilities, food etc, so thanks for confirming that. I do drive but I want to ensure that should anything happen in the future I'm not stuck to a twice-a-day bus route.
A Covid plus, since March I've cleared out the massive loft and bedrooms over weekends where I've usually been too exhausted to move after a full week at work.
Your feedback has been great, thank you. All suggestions welcomed.