Take it to a radiator repair shop and they can repair it just like the radiator. I had mine done for about $25.
My radiator overflow tank, the newer style, was sent to a powder coater to put in his test oven to see if it would handle the temps required in powder coating. Alas, it would not--and a lot of solder leaked out.
Like Brad Badali, I took mine to a radiator shop, and they repaired it and pressure tested it. Then my local shop painted it the proper black. Was a bit more than $25 by the time I was done.
I'm sure, Brennie, that there are some radiator shops around. If you are in London, call our member stickandrudderman (Colin Ferns) who is on the SW part of London, surely he knows of a radiator shop in the greater London area.