Went to the vet today and he said it was very unlikely to be medical, but gave us a urine sampling kit anyway. He thinks it is behavioural and due to stress and gave me some websites to check out and some tips. The vet assistants there said it may even be hormonal as she is coming of age. She is being neutered in two weeks.
Last night I moved her litter downstairs, and lo and behold she has been using it to pee in several times today, and even went into the bigger litter box to pee earlier! She still peed on the couch a tiny bit this morning, but no big puddles/stains like before and nothing since then. I hadn't moved it earlier as I thought it would confuse her, but clearly she didn't like it in that room for some reason. So fingers crossed now! Thanks for all the great advice here, I thought I would go mad the last week, but there is light at the end of the tunnel and no laundry today :D