Hi Pete i've just done the de-crudding of my rear arches too, once i'd dug them out I gave them a scrub around with a piece of that green plastic pan scourer stuff, then I soaked a piece of rag with some thick gloopy 90w gear oil which i happened to have lying around in the garage and applied a good layer into the inner ledge, i did two applications giving each one about 15 minutes to soak in, then finished off with a handful of grease (wearing those condom gloves) and gave that a good smear around leaving about half an inch of it around the inner ledge.

Its a good idea to stick a couple of carrier bags or rags over the tops of the rear tyres while you do it, but I escaped with just a few random drips.

Ongoing i'll scoop the grease out out and re-apply some fresh stuff every year and hopefully that should look after those area's.