Doh!!!! Now I feel like an idiot!
The reason it sounded like a rattle in the mudguard is because one of the mountings came loose! The ominous misfire on the way home was the fuel running low and needing reserve.
I fixed the mudguard and restarted it on reserve but one rattly sound from the engine was still there. I eventually noticed that the neutral finder was pointing at 3 - I thought I'd kicked it to N but it was in a false neutral. The ball bearing in a tin sound that I heard when I pulled up in my garage was the neutral finder mechanism rattling on the gears. Kicked it to neutral properly and the noise went away.
I guess I took 1 + 1 and made it equal 11. Just 3 separate issues which, up on that frozen heath, got concocted into something more serious.
On a plus note I know that my pushrods are correctly set and the plug is a nice nut brown and I now have a set of gaskets for the next "emergency"
Thanks all for their suggestions and pat yourself on the back if you suggested bodywork