john brook wrote:
Got some black dirt/grease off the car, on to my good trousers. The
usual trick which solves it, is to work in some Swarfega hand
cleaner; and then detergent and warm water washes it away.

My problem is that being in a hurry i did not work the Swarfega in
deep enough or well enough, and then went on to wash it.

So its now 'set' into the trousers. Grateful for any further
suggestions.

Repeat what you did, but let it "soak" the hand cleaner in overnight.