"It sounds mad but could it be changing the living room around? We put up the tree and got a new sofa 2 weeks ago and it's all been since then."
I would say you have answered your own question 
Cats are peculiar little creatures who need to feel in control of their environment. Any change, such as moving furniture, decorating, even a new air freshener/cleaning product can cause a cat to feel they have lost control. This leads to stress, which then leads to inappropriate behaviour such as peeing where they shouldn't.
What have you used to clean the urine up? A good way to remove all odours is to clean the area with a biological washing powder solution (helps to breakdown all the urine enzymes), leave the area to dry, and and then go over with surgical spirit. This should remove all urine smells, and encourage the cat to not use that area as a toilet. Most normal cleaning products won't be strong enough to break down the enzymes, and anything with ammonia in will still smell like urine to your cat.
Get a Feliway plug in and keep it on 24/7 (they are safe to leave on all the time). You could also try a product called Pet Remedy, which is valerian based (Feliway is pheromone based), and is available in a spray,and I think a plug-in.
Did your cat have a bed in the living room? Is so, and you've moved it to make way for the sofa or tree, this is likely to be making him/her stress out also.
Take a look at www.icatcare.org
This is the website of the International Cat Care society, and they have heaps of useful advice (and handy videos). 