Thanks, a few ideas. I've been very kindly offered some surplus seat cushions for the cost of postage so that's given me some options to keep things going for a while
I might drop Quayside a line and explore the possibilities of getting some made up using a decent grade of material and waterproof bases. I don't know what the military etc use but it must be fairly tough stuff.
None of my seats are hinged so hinge failure is not really an issue - they hook under at the back and the clamp at the front is an over-centre latching arrangement. The hinges on my double jockey Humber seat I bought and installed on the Destroyer lasted less than a year so I wouldn't bother using hinges again! It's actually more convenient to be able to take it right off anyway.
Has anybody pulled an Osprey seat base to bits? Is it just a flat bit of ply with foam on top or is there a structure inside?
I suppose what I am trying to ask is - is it practical to un-staple the upholstery, take off the base and treat it and then refit it, or is the whole thing going to fall to bits and/or look like a dogs breakfast afterwards?