Hi Ian, I’m in the states so can’t help in person. But I have a few suggestions to offer.
If it’s running rich at idle and lean at load my first suspect would be leaking spindle bushes. There’s two ways of testing this:
1) Grab the spindle ends and wriggle them. If you feel movement that’s a lot of wear.
2) If you can get the car running spray starter fluid or carb cleaner on the spindle ends where they enter the carb bodies. If the engine speed changes that’s another indication that they need help.
You most likely have HS8 carbs on your car. The spindles can be replaced easily. However, if the wear isn’t limited to the brass spindles you may have to have them rebushed by a specialist. You can remove the carbs yourself and send to a company such as Southern Carb services for example.
You can also check for intake leaks using the spray of carb cleaner method. Check between the carbs and the intake manifold where there are large o-rings, if the o-rings have aged they can leak too.
I would also check float height:
Remove covers from float chambers, retrieve floats and shake. There shouldn’t be any noise if you hear petrol sloshing inside they have started to fail and will need replacing. Heavier floats equal more fuel going into the carbs.
Sounds as though you’ve already checked and replaced damaged needles. Did you also check the jets?