That little lot gives me some useful suggestions, thanks!
I think I must have been having a Doh! moment not to think of just slapping some reinforcing in underneath - I ought to have a look at what is required for seatbelt reinforcement. Isolation I think I can do with a suitable interface layer - even WaxOyl would be better than nothing, but I've got some interesting epoxy paint that might suit.
As for B&Q's metal selection - well, they sell the stuff in manageable quantities, but... Most of it is hot rolled stuff, from what I remember, and quite pricy (not to mention never in the sizes I want!). I don't remember seeing anything in stainless there - and if they do, can you imagine trying to find out *what* stainless it was? "Hello, is this a ferritic or austenitic stainless?" Argh. Actually, that one would be an easy one to find out.
I could probably forge something interesting to do the job...
Thanks again,
Peter