Forgive my ignorance, but is the idle jet the little nedle thing that screws into the front of the carb? has an offset slot in it and a collar around it?

That fell out of my yellow car once, and caused the symptoms you gave.

Russell,
nope, that's the idle mixture screw, turn it inwards for leaner, outwards for richer...

Jono,
I fitted a replacement engine for a customer today. Bit of a rush job, as he'd contacted another place first about his engine being noisy and they'd advised him to run it until it expired...
When I pulled the sump plug to have a quick check, one of the 3 springs from the camshaft wheels was stuck to the magnet.

Anyhow, we ran the prospective replacement engine on the bench before putting it in the car and it ran perfectly, ticking over like clockwork.
The customer asked me to swap over his original manifold with carb, whereupon it ran like a bag of spanners, tickover uneven and juddering, with hardly any response to the mixture screw.

The 'small oil can filled with petrol' trick found the reason very quickly, an air leak at the carb base.
Maybe worth a try on your engine, checking the inlet manifold to head joints as well, since one bolt slightly loose will produce enough of an air leak to affect the tickover.

ken

I changed the carb this morning for another random Dyane one that I found in the back of the garage. All seems well again now

You been syphoning petrol from dirty tanks Jono?

Ken's right, I struggled to find a similar problem on a customers car recently and eventually discovered a minute crack in the manifold. Changing the carb might have just closed a crack temporarily.

That's interesting, thank you Paul and Ken. It is running perfectly now, but I will bear that in mind and investigate it some more as I couldn't really understand why both previous carbs had ended up with the same problem.