the JBWeld (or equivalent) sounds like the easiest, and the valve stem sounds very good and maybe more reliable because it sounds like it would tend to be self-sealing from the pressure but mroe trouble of course.
Knowing me I might go for the easy one but I'll see how it pans out.
Thanks for the great collective response, guess it isn't such an obscure issue after all.
Another way to prevent it in future would be to secure the hose to the cavity wall with a bracket or two. A long hose looping down might fill with water and inhibit the gas movement perhaps, probably not though. I'm thinking its main purpose is to allow expulsion of gas that may build up in the radiator, in which case it probably isn't an urgent thing. The bigger lower hose would be the main one for coolant and pressure.

PS drove it tonight and it ran fine - hopefully charging is fixed also.