[Chris152]

Thanks both, and yes, I'd forgotten about crumple zones so we'll try to proceed with plan A.
The welder says fine, and I've just got prices for the two sheets of mild steel (apparently they can't get hold of the zinc/ zintec at the moment, supply issues). £35 for the steel, and £10 for a 30mm (external) x 1.2m pipe to wrap it round for the internal profile (seems annoying but I can't find the right size laying around - and 10 isn't bad to be able to get on).

But - they don't do 1mm sheets, either 0.9 or 1.2. I prefer the idea of 0.9mm as it'll be easier to shape, it looks like turning a meter of steel through 90 degrees isn't that easy, and 0.3mm could make difference between a tidy and botched job! (The original measures 1.1mm, with paint). Are we ok to go ahead with 0.9mm? If I put the order in today or tomorrow morning, we can collect Friday.
Thanks, C

ps As a bonus, the chap doing the welding is going to let my lad watch/ help and he's going to talk him though and show him what he's doing. :-)