william I was also taught to use stag in my apprenticeship days. The product Stag has been around for longer than I can remember (and these days it is not that long) and to my knowledge the the formula has not changed so it must work. It is primarily used for water and steam jointing by plumbers and boiler attendants and does not cost a fortune.
242GT Alex, if you can, please put up a photo of the leaking welch plugs, the ones I used to buy and install where shaped like a frizbee.
ab1 https://zeligsgarage.com/products/volvo-red-block-freeze-plug-retainers-blank-blue-anodized not sure if these are the correct fit for a b30 but seems to look like somewhat of a permanent solution to the problem.