Thanks chaps for your input , i have seen that piece of armoured glass before it might be from a  Marauder rear turret but i cant  be for certain , i will have to so some research .

As for Tettenhall , yep i went there and took loads of pictures and measurements but i dont remember seeing any thing to answer my question but never mind i will be calling again soon along with the RAFs example at Cosford .Hopefully between the two i can work it out ?