Thanks for your replies.
Hithere This isn’t a new problem as such. She was unwell but after lots of tests etc, she is now stable. It’s been difficult to get her to go out again since being unwell.
However, I think I might seek advice from the vets.
I know it’s her soiling, as DP’s two cats are around 17 now (brothers, both neutered years ago) and spend most of their time asleep. They were asleep on the bed with me and had been for hours, when the latest ‘bath poo’ was discovered by DP.
grandtanteJE65 - one of DP’s cats has started showing signs of dementia
Apart from the downstairs rooms, the upstairs living area is open plan.
Short of locking her back up in the bedroom, there’s nowhere for her to have a ‘private’ litter tray.
We really want to encourage her outside as we would prefer to get rid of the tray. She never had a litter tray when I lived alone with her.
septimia - we do suspect something spooked her. She was just getting used to going out (before she became unwell) but she’s so reluctant to go out now. There are around 7 cats living in this building (it’s a converted very old building with 6 dwellings)
I don’t think DP’s cats are guarding the cat flap. They sleep so much and are really not too bothered about her.
She would love the vast amount of space there is to explore, if only she would go out. She spent so much time outdoors before we moved here.
She seems really settled in the house now, but we just cannot overcome this soiling business 
I think I’ll take her back to the vets again soon but in the meantime we have decided to brave the cold when we are in, and keep the front door open. At least we ‘live’ on the first floor so it shouldn’t be too bad!