Thanks for the input - I sorted it out and it worked perfect!
I used a German product called Vanish for this, poured some of it in a spray bottle and diluted it with a very small amount of water. Blasted the seat and rubbed it off with a white cloth. This stuff is so strong, it even turned my hands white and I can't wash it off now. White cloth turned blue.
Then I took my pressure washer and blasted the daylights out of the seat. Followed this up with my wet/dry vacuum and now I've left it to dry. Worked a charm, but my hands aren't very happy.