The rudder's mended. My metals friend took it, welded it, then sliced off the extra thick layers of Al from the top end (the part inside the stock) and replaced them with fresh Al in a longer length, so they extend downwards over the fracture. All welded together very neatly. So, once re-assembled, the visible part of the blade has about an inch of extra metal at the top as it disappears into the stock. Hope that makes sense to the reader!
I scraped off the old antifoul (the stuff not suitable for aluminium) which hasn't helped the corrosion [:(]
I primed it and put on three coats of Trilux, re-hung it, it looks very smart and we went for a sail today and it feels fine.
Over the winter I think I will get a new one sorted. Still getting quotes for a custom Al blade, and I did briefly succeed in contacting Bruce Parker who indicated he could sell me a P235 rudder but I've since lost contact again.
Good to have the boat back in use though!