Radweld

Does that mean that Radweld has replaced an egg whites 🤔

Transferring heat from the combustion process through the head into the coolant present real challenges, which with the development of modern engines, has only increased. The very thought of adding a coagulant into that challenged situation, can't be anything but a "bodge".

Here it appears we are also talking about a tow car so one facing sustained over loading relative to solo use, in itself adding considerably to the heat transfer demands being faced.

If I could not afford to do a proper job immediately, I would add distilled or deionised water, way before adding anything else to the coolant. At least the waterways in the head will not be compromised by whatever the coagulant throws out, so now and after a proper repair the heat transfer performance will not be compromised, now or forever.

Professional radiator repairers repair them for a living, I would seek a quote there.

Nearly any independent car mechanic is likely to run at overheads way lower than a main "dealer", so if the change out can't be DIY or the radiator specialist, that is where I would seek a quotation for that part of the labour cost. 

I know a bit having owned and run various classic cars over the years including some with marginal cooling systems like the Triumph Stag. All I can say is that if I visited your business I’d do two things… 1) take the radiator cap off and look for oil in the coolant indicating head gasket failure and 2) check the colour of the coolant. If it was a clear green, pink or blue colour I'd be happy. If was dirty brown indicating neglect or Radweld in the system, I’d walk away. If necessary for a quick temporary fix I’d subsequently drain and back flush the system to get rid of all traces of Radweld asap.