It certainly sounds like a leaking heater matrix, its unlikely radweld will cure it without causing a blockage or restriction. If you get a Haynes manual it will tell you how to replace it. The job varies from car to car, on the Citroen Xantia the whole dashboard has to be removed but on the XM you can access it quite easily. You can as a temporory measure bypass it by joining the two hoses together - if you don't feel the cold that is.

It needs diagnosing properly, don't assume it is the heater radiator, it could just as easily be a perished or chafed pipe, loose pipe connection or indeed the heater control valve leaking.

If the car was a VW group effort then heater rad is a nightmare of the first order like other jobs that should be straightforward are, but some cars are not too bad.

I wouldn't be stuffing sealeant by Bodgit and Scarper in my coolant if i was intending to keep a car.