Same used to happen with me its because you have probably been using the hand break: we were told this makes it sieze up. Instead use bricks if it rolls away.
Anyway, lie underneath and pull the metal rods that attach the wheels to bottom etc... im not a mechanic so crap at explaining: it will just loosen it all up again!!
Hitting the wheel can be successful, but the damage that can occur is unseen....the brake linings are torn off the shoe inside the drum. Effectively, should this happen, your braking could be compromised.
A lot of people know not to keep the handbrake on while it's not being used to stop seizing, but a lot of people don't know that when you reverse to park it you must pull forward again before unhitching it, otherwise you'll leave the brakes on, which sounds like whats happened here.
Thanks guys managed to get it free. It was used mid jan but my partner parked it and he probably didn't pull forward again as suggested. We'll know for next time!