Russ
There is only one real solution: replace it.};-)
If there is a pinhole then it has corroded away inside. Its like an iceberg the bit you see isn't the half of it. There is a stuff called radweld you can try, it works like a repair, put it into the make up tank and bleed some water through the towel rail while its warm to draw the stuff to where you want it most. This will buy you time to get some quotes and budget the repair from a respectable plumber. remember the next leak won't be a wee one you'll end up with wet socks and an insurance claim so don't "mook abaght!"
Get your plumber to look at whether the system is correctly earthed and ask him to check for galvanic corrosion this can sometimed be dealt with by running a wire across the copper pipes either side of a cast radiator. the rad acts like a battery circuit because of the different metals (a copper / iron galvanic bridge) you see.
kennyf, March 2009
