Thank you for the suggestions, I shall investigate further with OS maps and maybe a day drip to check out the lie of the land locally. For future trips, the Brecon Beacons are reasonably familiar, but I've never ventured far into Dartmoor so would be worth a visit.
I picked up some ideas from a recent Trail magazine, and I was "encouraged" by the 2 hour walk-in to the bivi site which is generally more than most dog-walkers would do for a late night constitutional.
I've never slept outside of a tent (apart from the occasional alcohol-induced event as a student, but that's nearly 40 years ago now!) so don't expect to get much restful sleep the first couple of times until I become more practiced. And I'm not sure how much insulation I'll need (bivi bag, sleeping bag, mat, down jacket, etc) Hence the need to be close to home.
The warning about lights is good, I was thinking of going on a Moon-free night to watch the stars, but maybe a bit of natural light would be a benefit. Or shelter in trees. I'm not keen on being directly UNDER trees having had a branch fall on my tent while I was out walking!
Thanks all!