OK, I've managed to get out for a quick look and measure up this afternoon. The right-hand post has come away due to rot at its base and its fixings having given up the ghost, but it's been wedged and can be opened/closed with care. I've also noticed that the bar the gate hangs from is badly corroded where it goes into the posts. The lintel is sound and the posts aren't attached to it so I think it's fine to leave as is and just replace the uprights.
I've got 2m of 100x50mm box section in my garage that will do the job for uprights and I'll see if I've got a suitably long length of 10 or 12mm round bar to replace the rusted one. I've got some bolt resin that I think is still OK but heavy duty angle would be useful for top/bottom support (what section size is it, Ric?) and loan of the DCA cordless grinder may be useful; my 24v Hitachi should be fine for drilling fix points as long as I remember to charge the battery. :-[
Thank you everyone who's offered help so far, I'll post up here to sort out a date once I've got the replacements prepared. I'll also see if I can find the spare gas strut I've got somewhere and see about fitting that to the gate to ease opening and closing.