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Hi Jack . . Recently on a journey into the countryside I came across a long patch of road which had only recently had diesel spread on it to keep dust down. I now have diesel "muck" under four wheel arches. I have tried to hose this off to no avail. Will this cause damage to my undersides or should I just leave it? Or do you know of a way to get rid of it?

This is highly unlikelyto be diesel. Diesel is an oil based product, so diesel on the road will cause vehicles to slide out of control. Also, it come off relatively easily with a high pressure hose.
It sounds more like bitumen or tar, which can be very difficult to remove. Usually a solvent based product such as white spirit or similar is the only way to remove tar - and unfortunately it requires a great deal of elbow grease too!
Other than being visually unsightly, it is unlikely to cause damage.