Put all tinware back on. It was struggling to idle. Went to put it back in the workshop so I could tackle that aspect tomorrow and then it was idling fine. So I thought I would take that opportunity to take a shot run 100m to the roundabout up the hill and back to test for backfires. 30m out the gates and knocking out of nowhere. no oil anywhere, smells a bit fishy in the engine compartment. Got back in, had to start it to turn her round. Limped back to workshop so as not to add to the damage. Here's a video of the noise.