It's only been happening for just over a week and only on about three occasions. There have been a few changes here, like Phot going away and also our other cat of her age had begun leaping on her and biting her neck, when she obviously wasn't interested in his attentions. He's now been neutered so that's no longer a problem, so I wondered if she'll now settle down. She doesn't have a litter tray anymore, since they started going outside at about three months old they preferred not to use it anyway. I stopped putting one out as it seemed to encourage stray cats to come in and mark the area.

I think the Guest above makes a good point about her maybe picking up on my stress. Thanks all for your advice, certainly a vet trip will be in order if this doesn't stop over the next few days