Yeah I guess it wass time to pull the bandaid and have a proper look.
The outside of the pad was covered in black goo and hard to measure whike installed, so I decided to pull off the pad entirely.
And it was not what I was extecting - like RBS suggests the whole thing it's somewhere around 1mm (don't have calipers for more precision), but the black goo that was on it was hiding a nice thick copper block and the actual thermal pad is very thin - thicker than a couple of sheets of paper but not by much.
I have cleaned the copper and am going to try to attach the cooler directly to the GPU IHS with some Artic MX-4 between and see where that gets me - if too hot I'll have to buy a very thin pad instead
Thanks for the help guys.
Thinkpads: Too many. Currently playing with an IBM 535