Right, problem solved - almost. Took the manifold off and filed away the bit of the bolt that was protruding through underneath - bolted everything back up, and - instant start up. It's closed the gap that had opened massively, but it is still there, just.
Now I need a new manifold as this one is a bit damaged from where the bolt hit it, and a spare bolt. Oh and my engine management light has come on.. I assume this is related to unplugging the gubbins on top of the manifold? Just that it's not really showing anything on my code reader.