How do I get it out!? HELP

Hi I just thought I'd change my engine coolant temperature sensor to see if it improved a minor running issue. Took old one out and screwed in new one which has a plastic nut top, it didn't seem to Tighten up correctly so I tried unscrewing it but now the stupid plastic nut casing just rotates on the internal brass sensor and won't unscrew from the housing. Anyone had a similar problem and most importantly how did you manage to get it out?
V8 4.2EFI

Cheap nasty brit part sensors, I was sold one a few years back and got a refund cause I had the same issue as you previous.

It's not an easy resolveI ended up with the stilsons on there but you may be able to get a chisel in and knock it round, there ain't a lot of room.

Thanks for your reply Pete! I've just been under the bonnet fighting with it this morning, I had to be really careful not to get mad with it and damage the manifold getting it out. (Had to walk away once and get a cuppa and some biscuits)
I managed to get it out without damaging anything thankfully. A small set of mole grips (on almost crush setting) gripping the very lowest part of the plastic casing did the trick!
I have no idea what was up with the new part, it didn't screw down right in the first place and it didn't even seal as it started to leak from the thread.
I'll always go for the brass nut one in future.
I must admit there was a moment of absolute horror when it just seemed to turn it the housing I thought I'd stripped the thread in the manifold!!!