So, having for some time thought that my loom didn't look quite right and might not be the proper shape, and then I after getting irritated during the last piece of weaving by the upper jacks sliding forwards so the shafts were catching on each other, I got out a spirit level.
There was a difference of a few millimetres in height between the back and front of the support for the jacks. For the last length of warp I used a minor modification - an old birthday card folded up to lift the front support.

Then it was time to take a proper look at the loom, with set square and tape measure.

It just so happens that last week we had a large delivery of sycamore wood for building a new staircase. The woodyard also sent the offcuts, which included some good sized pieces of wood. The resident woodworker got to work for me with his power saw, hand plane and router. The new supports were then finished with a couple of coats of danish oil. They look lovely. It's not the same as Finish birch (as used by Toika) but the wood is pale, close grained, strong and beautiful.
See how square the corners look now:

The posts were fitted with simple plates to make them "I" section that are screwed into the frame top and bottom.

See below for the tenon joint that holds the fixing plate:

Just to follow up the literary quote in the first line, this is from "How to be Topp" by Geoffrey Williams & Ronald Searle, first published 1954, the copy in our house is a well read and loved falling-apart Puffin edition of 1964. Since I discovered this book a few weeks ago our tabby cat Pheobe is at times nicknamed "molesworth" for her quick intelligence, clear no-nonsense view of life, imaginative adventures, and habit of "toughing up" lil' Annie cat, a.k.a "molesworth 2". Jaspar, the smart neutered-male ginger cat over the road is "Basil Fotherington-Thomas" - who skips around saying "Hello clouds, hello sky" and sometimes gets invited to join molesworth's adventures.
(See this post of July for an introduction to Annie & Phoebe).
(See here for Wikipedia entry on Nigel Molesworth.)
(Just to make sure this is on the record, my loom is a Toika Norjaana 8 shaft countermarch, and I love it because it is beautiful, has a good strong frame, and I can use it for all sorts of weaving - and it will take modifications!)