Well I have just had the manifold off, it all looked ok - no obvious signs of a blow. Put it all back together with new gasket goo and it's exactly the same
I have double checked everything and can not see anything wrong, all connectors have been off and reseated, the iscv wiring checks out but something has changed since that backfire. It was ticking over at about 850 before - steady as a rock.
I have checked and double checked the pipe to the breather, the one to the MAP sensor, the one from the throttle body to the regulator, the one from the manifold to the iscv and the ones that are blanked off, they all look secure and intact.
It's got me beaten at the moment, don't know what else to try or where to look.
Beer. That's what I need. Lots of beer.
Thanks for your input guys, if you get any other ideas please let me know. I will look at getting the proper gasket - thanks for that, I tried everywhere local and came up with nothing.