Blood work was done and came back as normal. I will however go back and get it done again after xmas, just in case anything has changed.

Since moving the litter tray back into the flat it has stopped the problem. She no longer goes just anywhere she goes in the tray.
I guess she was just being picky and decided she didn't want to go outside in the cold any more. (probably her age) The few years previous the tray was inside all year round so I think she was used to it being inside.