Thanks for all your helpful suggestions.
In the end I bought a pair of new pedals from Wilkinsons, replaced one and swapped over the cage on the other one. luckily the mounting screws were in the same place!
To finish the project I need to source some cantilever brakes. I fitted some that I had lying around but they are a complete mess and they don't work (see pics).
I don't want to spend a fortune, as the bike cost me 99p on ebay!