I'm hoping it's just the slider has seized, they're common for doing it. With only doing around 1500 miles the last year and garage it's not surprising. Ordered some brake cleaner so will do it over the weekend hopefully.
Probably need to charge the battery too, going down the road and back after jump starting it wouldn't of charged it much!
I put it on a slow charge last night including reconditioning it. It took about 16 hours in total but should be good to go now.
When I removed the battery there was a puddle underneath the tray again! Barely used the car so it's not being forced in through driving. It's getting in from the rear of the boot somewhere as I saw a line of water coming out from under the trim. I think it's the boot lid seal but I'm not sure how to test that. I could try getting a used one to test but that might give the same problem.
It was too cold and wet to look at the brake today. It'll probably have to wait until the end of next week now.