New cylinder head fitted! Back on the road!
It was £500 from Roughtrax - don't know what the garage bill will be for fitting it yet but I took it out for a long drive on Saturday. All seems back to normal - hurray, we have missed the van. At the same time the radiator and rear heating were flushed out. Hope that solves whatever caused the cylinder head to crack.
In the process of this I learned that my engine is not KZ1T but 3L is actually the engine series. Any thoughts/info on the drawbacks (and advantages if any) of this engine, things to watch for, etc?