Thanks for the comments. The steel cost £320, all in, including laser cutting, from laserprofiles.co.uk in Wimborne, Dorset (who were excellent - helpful and prompt).
So no, not cheap. Then again a replacement new mild steel deck with indifferent power coating is £400-£450, so I have saved a bit.
Mind you, I am retired and projects like this are just a hobby. If I had charged myself out at even miminum wage rates... well, let's just say I am a very slow worker and it took quite a few hours.
Question for the Great Welding/Fabbing Sages.... (I have a Countax at my folks place that needs tidying up)
If the deck was repaired and the underside cleaned.... could you coat the underside and edges with glassfibre - and would this provide s decent corrosion barrier?
I think the fiberglass would be too brittle and probably delaminate under the vibration and the hail of shrapnel from the blades but might be worth a try.
About 10 years ago I bought a ride on that had the deck welded up.
I blasted it and had it galvanised.
I never clean it or even take the worst of the caked grass off it from one year to the next.
I had always intended painting the deck as well, but am just getting around to it now.
I pressure washed it and gave it a going over with a wire brush. (The galv is certainly getting used up in places.)
I will sweep blast it, give it a couple of coats of Jotun 90A then, for an experiment, do one half with glass flake epoxy and the other half with single pack stone chip. (Sikaguard 6440)
I have zinc rich epoxy but don't think it will stick well enough in this application which is why I'm going with the epoxy mastic.
I'll tell you which is best in a few years, but I can say hot dip galv does work.
Any normal person would have scrapped this thing vears ago but I intend to keep it running as long as possible and it's getting it's second paint job (and a colour change) to tidy it up.