thanks all!
Have booked her in at the vet on saturday, they think it could be blocked anal glands as has been mentioned. She left us a nice poo to get out of the tray this morning so that seems to be working ok. Weirdly she only tends to do it just after she's had a sleep in the evening, maybe she's just too lazy to lick her bum when she's just woken up? HAHA!
Poor little thing, dreading taking her to the vets, just when she's finally settled here.