Thanks for the info. I will check which is the flatter flange side when i get a chance.
The other side of the gasket isn't completly flat, its got a very small raised bit, but its tiny in comparison.
I spent 2 hours getting the midsection off my 2008 GP. It was the original midsection but a 2019 backbox. The midsection hadn't actually failed it was rusted to the backbox so the whole lot will need to be replaced.