Hello all, sorry if this is me just being a bit thick.
I purchased a TX-P42GT30B and got two pairs
of 3d glasses, which worked fine.My infant son
broke the screen and the insurance replaced
my TV with TX-P42GT50B ( lucky me ), but I
have never been able to get the 3d glasses
working, everything looks right on the TV side.
Glasses have been fully charged.
Any help or suggestions would be much appreciated.

I think you'll need new 3D glasses. As far as i can tell, the GT50 uses the newer Bluetooth controlled glasses, while the GT30 uses the infra red type.