how about turps ?

White spirits or lighter fluid will do the job just fine. Based on cost alone go with white spirits, lighter fluid is basically only white spirit anyway.

Thanks Gents. I'll try a few tomorrow. Really pi..ed off at myself for being careless.

I have to disappoint you, you can not remove diesel sludge marks from material, completely.
It is because the soot and the glicerine in the sludge. You can keep washing it and washing it but you never ever be able to get rid of it completely. Sorry!

l would - while kicking myself - turn around the cushion the other way, and get busy with the fridge !

White spirit and turps don't work. Neither does tea tree or eucalyptus oil.
Engine sump oil is hard to shift.
I can reverse the cushion with the other bunk. Some stains will still be visible but from experience such stains do fade over time. I had a small engine oil stain on a cushion years ago, you can't see it now.

Still kicking myself SirG. But I had a nice bottle of wine last night to console myself.

On the positive side, adjusting the tappet clearances has made the engine start and run better. An often neglected part of a service. Only needs to be done every few hundred hours but very important for engine performance. And the new fuel filters and engine oil will see me through a couple of years before the next service.

We are talking about D I E S E L engines here!

The nature of those engines, they create sooty deposits in the sump oil and also create glycerine in the sump oil, somehow, by not completely burning the diesel fuel. Once l had the thing explained to me by a science guru but l forgot. I am not a chemist.

Whoever, when this concoction gets into materials it is - unfortunately - next to impossible to remove, completely.
A pair of scissors is the only tool, one recommended to me.

For twenty plus years l was involved with transport, using diesel powered machinery. We always had issues with sump oil stains on concrete, never to be cleaned by anything. Hot, high pressure washing with detergent has done the most damage to it at high cost, still visible, though.

One could try high pressure washing it after soaking it in WD40 for a while.

The better half suggests a paste made with baking soda. Apply the paste and leave for a few hours / overnight.