Thanks very much for the replies, apologies for disappearing.
I've spoken to a Challenge dealer who's going to see if he can get a response from them about a replacement rear hanger plate (I think it might be easier to make one myself, but we'll give them a chance).
The Terracycle suggestion seems very sensible, but I tried to order from them a little while ago with mixed success. I emailed Pat Franz asking for confirmation on which particular model I should order. What followed was about the most comprehensive customer service I've ever experienced. Emails whistling back and forth with photographs, dimensions and engineering sketches. He settled on exactly what was required, I said "yes please, front and rear, in titanium, let me know how much you want", and then never heard anything further.
I'll follow up with him again, see if my order got lost or forgotten somehow. The contact I had with him was hugely impressive, right up until I said take my money. Ho humm.
Thanks again.