If I don't drive my car for a few hours it's a 50% chance I get a park brake fault, if I run in and out of a store it never happens. But it's almost guaranteed jammed over night. I still have the stock jag battery from 2006. My car has 40k miles, is switching the battery the easiest fix? Or it's more than that? I'm getting annoyed of it.

 

Yes, check the voltage of your battery first.
If it does not hold the charge, it's probably done.
If 10 years old, already you are lucky that it lasted so long.

 

I would pull the fuse pro tem, you can drive around without it until you sort it out.

Fuse list in owners manual.

I am guessing that the problem is more than just battery.

 

I would lean to a battery issue considering the age. I had the parking brake fault both on my current XJ8L and my previous S-Type. A new battery fixed it both times. Jags are real picky if the batteries aren't up to snuff I've found.