Yeah, I've spent the evening checking for the likelihood of it being a LH thread - because some are. The Rhona site lists replacement and alternative inlet pipes and quotes them as 1/4" NPT m, not a mention of LHandedness, so I'll have at it again tomorrow. The nut the fire guy gave me was for exactly this purpose, after I told him that I'd sourced the reg from a European provider, and he suggested I have a matching nut in case I needed it. If that inlet tube had come out easily, I'd have the new nut on and been using it today. The replacement nut seems beefy enough, and looking at the reg's own nut, while it's quite beefy, the retaining metal at the clamping shoulder doesn't really amount to much - much of the clamping shoulder has been removed by forming a hex on it, for the purpose of unscrewing the pipe. But by the time I fettle the new nut to closely fit the stem, there will be a better fit.
One good thing though; I found the body of the reg is solid forged brass, not cheap casting, so it should be man enough for a bit of heave-ho, but if it shows no sign of moving I'll just halt. No point in wrecking it, as it's definitely not a cheap piece of rubbish.
I think these ones were sold as the end of a run of ones fitted with 1/2 BSPP and probably punted out the factory door cheaply, just to get rid. Hence it found its way onto ebay and I bought it from a Hungarian chap on Ebay UK.
I wish I'd noticed before now, the difference in size.
I'll ask the fire guy if he can supply the next one with a simple handwheel and outlet thread, good idea, thanks.
The stem should have a normal RH thread but stems on regs for fuel gas like propane & acetylene are a LH thread.