Thank you for your advice, gentlemen!
I feared that after some 63 years in place it would be more difficult to remove than it actually was but with a sleeve and cheap threaded rod it came out quite easily and smoothly. The most difficult part was finding a suitable sleeve, but seeing a length of water pipe I'd used as a tree stake in the garden I saw that water pipe screwed unions would do the trick. So above you can see a "3/4 inch Black Malleable Iron Socket" in action; it cost £1.03 at a local plumbers' store who still stocked such stuff. (Woodstock Plumbing, South Way, Walworth Ind. Est. Andover SP 10 5BN - I should have bought two to get the proper length.)
The threaded rod is cheap 8mm metric DIY stuff and the nut on the end in the valve chest needed its corners ground down to pass through the hole in the barrel.
Again, thanks from MikeN.