Thanks for the replies peeps.
I did check the deck and i'm pretty sure it is a previous paint colour, not dirt (although I haven't put the water pressure washer on it as yet). On other areas you can see the paint where it wasn't masked well. See the pics below, I'm not sure what to make of it.
I'm still looking for peoples feedback with Interdeck paint? Anyone ... ?
I may just have to pony up and give it a go.