If your not confident with pressure washer get a hope pipe with attachment and just reduce the flow right down
It's ok, I went ahead with some GUNK engine degreaser and scrubbed the worst areas with a small brush before rinsing with the washer. All seems to be ok, touch wood.
Watch where the run-off goes - don't let it get anywhere a cyclists / motorcyclist might fall off.
I was doing it on the drive and moved the car to a position that the water would run out of the way so no immediate danger to road users.
Maybe if hes single :) id love to see wifes face when trying to insert oil soaking liner into a washing machine..
Haha, yeah I chose not to go with the washing machine and instead applied a good quantity of sodium laureth sulphate and scrubbed it by hand.
Incidentally, I think the bonnet liner has been oil soaked before, possibly through the same error, as the area near the filler cap is in very poor condition on the underside and has never fit properly along the back. That plus the slight brown staining it had already caused to the engine cover, something I'd known of since we got the car but never investigated.
Anyway, here is the after shot (the spots are rain); just need to dry the bonnet liner and refit but otherwise it seems to have gone well.
Thanks for all the replies.