So, having found a radiator with the water retaining capabilities of a colander, it was time to take a visit down to the local Radiator specialist. I explained my predicament and he said it would be possible to recore the Allegro rad, but it wouldn't fit my budget as it would be £150. He was right. However, as I had taken down both of my old rads to show 1) what should be in the gap and b) what was ACTUALLY there when I bought the car, he was able to immediately identify the other one as that from an A-series Metro.
He disappeard upstairs and found a new/old stock one which he said had been "up there a while". He also knocked it down from £65 to £50 provided I paid cash. I have a new rad, I have supported an idependent local business and he gets to go down the pub. Everyone's happy! I will need to re-drill at least one hole as the mounting pins are in slightly different places, and I need to figure out how to mount the electric fan as that's different too, but I am nothing if not resourceful.
I had to go shopping for a Metro expansion tank & pipes though as I only had an enormous expansion tank from some sort of Ford in the bits box that came with the car. Ebay came up trumps so now I am currently making it a bit cleaner as it's a bit gungy.
New rad. Fills the hole although mounting points & fan will need fettling.

Expansion tank with pipes. Cleaning currently underway.
