That's not good but going back to what you originally said
It’s 2007 model with a raised bonnet, 5.5k on the clock and asking price is £20k. BUT! It’s an ex-demo car of a Nissan dealership. I’m a bit worried of the amount of abuse it has taken for these 5,500 miles.
as I suggested in previous posts, given the short time you have had the car and didn't get a chance to try the roof until recently I would still say you should take the matter up with the supplying dealer as selling you something 'not fit for purpose'. If the latest diagnosis is correct then your intial fear might be justified in that some clown (either a salesman or someone they have lent the car to for a test) has done the wrong thing.
I think you will find Nissan will not take responsibility now their tech department has opined that the problem arises from misuse, but your thoughts of involving a solicitor may be necessary if that supplying dealer tries to get out of a remedy.
That's my opinion.