Thanks for all your replies. Had a go last night with my pliers but they were only a small pair so no joy - of course it could just be my weak girly grip

!!
My plan is to pop to B & Q with a couple of screws in my pocket and have a sneaky try with some bigger pliers or a bolt cutter. I'll just try not to take anyones eye out with bits of flying screw!
Cheers
Kirstie