Bad running after new head gasket - cause?

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I wondered how on earth El Mecco managed to run the engine on three cylinders and direct the flow of the third injector into a yoghurt pot - the injector rail won't allow it. I would however work on a K-Jet system, which had a separate fuel line to each injector.

Why did the head gasket go in the first place? Head gaskets don't just fail because the engine is left ticking over.

Banging around in a field - The bottom of the fuel tank could have taken a big knock, has this upset the in-tank pump? The fuel lines sit under the car - has something happened here? Return fuel line damaged, for example? Unlikely to ever have been checked. Cat destroyed internally? They don't like knocks, apparently, although I've never had a problem with one.

Could a blocked cat generate enough back pressure and extra heat to overheat the engine on tickover and cook the head gasket in the first place?

I remember you gave me some useful pointers in my epic Caravan Brake Balancing thread from several years ago, this thread reminds me of mine. When you finally get it sorted it's wildly satisfying...