Are there no radiator repair companies in the UK any more, might be a choice worth investigating! SF radiators were without question well made and reasonably inexpensive, but sadly now gone. I would explore more avenues. I found the aluminium replacement radiator in my Roadster kept the running water temp much lower/cooler than the original rad, which may effect engine management systems.
IMHO it's simply not worth even attempting to fix the plastic side tank radiators used on the later Classics as per OP. They all seem to leak eventually and you would be simply putting good money after bad.
I had heard of the more efficient cooling of all aluminium rads causing issue on lower power 4/4's, ironically overheating due to increased hysteresis effect on thermostat. Simply cured I believe with lower opening temperature one. A small price to pay for a radiator that stays water tight.
