Good call on the ACME - the thought had crossed my mind, but I just assumed too expensive and a bit more of a hassle to come by. My assumption regarding the expense was wrong:

https://www.ebay.co.uk/itm/1-2-10-AC...-/382095182523

- that's about half the price of 1m of 1.5 pitch high tensile threaded rod - and I only need around 12", and this tap seems like good value:

https://www.ebay.co.uk/i/35213872052...7750313&crdt=0

If you dig a bit deeper, though, you'll see the rod's readily available in metric, but the taps aren't... Or if they are then they're a couple of hundred quid..!

Right, I'm faffing now. I've ordered a coarse 12mm tap (not in ignorance of your suggestion m_c - I looked at what I already had and what was going to give me the least amount of ball-ache going forward..) - I'll try it. If it's shite then I'll remake it so it's not shite. 👍

Wal.