Hi Chris,
As you’re in God’s county, Chris Isles at Doncaster is the guy you want for servicing. Very friendly, knowledgeable and reasonable prices.
I drive mine throughout winter unless there is snow on the roads however, 3 things if you want to take it off the road, pump the tyres to around 45psi, will help stop flat spots. Fill the tank with fresh fuel and get yourself a CTEK charger/maintainer.
As for water in the boot, I personally have not heard of anyone with this problem on an XK so worth check the seals around the boot and lights.
Hope this helps.