Thanks Nels. So it seems the primary jet could well be loose or dirty maybe. Interesting to hear the mixture is only adjustable by fitting different jets, didn't know that .

I wonder if I should take each jet out one by one and clean them. It looks so clean in there - there was some sediment though. Perhaps from the gasket deteriorating. Hmm.

I would have checked it out but I have some evil flu thing that I thought had gone away. Kicking my arse again, eesh.

The buggy needs to be in good working order next week as I'm working at another new place, so I do need to get on it. At least it's ok once on the motorway, lol.

sediment comes with the petrol normally.

Is it fixed yet?

No not fixed yet. Had nasty flu. I drove it today and it was a total arse to drive until as soon as it was warmed up and it suddenly perked up.
Think I might drive it to work tomorrow and look at it in my breaks.

Now that you have changed the gasket, perhaps a recap of the issues may be a good idea.

It starts instantly as usual, but then suddenly needs loads of choke to stay running. Then there's clouds of smoke. (lol).
It's a bitch to drive then because it wants to stall when you pull away.
As soon as it reaches a certain temperature it's as if something switches and it suddenly perks up. Until then it's a challenge to keep it running.

The odd puffs of smoke once warmed up, and flat spots/hesitation. Still charges down the motorway and plenty of pull (lol) when it's cooperating.

No petrol leakage as yet from the gasket.

Thinking about it, it's been coming this way for a little while, I noticed a little hesitation starting maybe two months ago. It's still very reliable though if you can keep it running until its warmed up.

I checked the carb securing nuts and its a newish gasket block thing.
The manifold bolts are tight. I changed the plugs and the old ones looked very healthy.

I'll try and check out that primary jet tomorrow.

Something to eliminate.
The choke pull off diaphragm may be perforated.
Standing in front of the car, on the left hand side of the carburetor and held down by 2 screws is where you will find it. The gadget is not necessary for the car to run and people do disable it.
There is a rod coming out of it toward the front on the carb. A rough and ready test is to smear a good blob of grease into the part where the rod disappears in to the gadget. Start the car and see if the grease is sucked in when you rev it a little. If so, the diaphragm is perforated.
To fix, unscrew screws, a bit of tape, then replace the plate.
Simples

Awesome tip, thanks!

Roger V, you may well have provided the solution to my problem.

Actually the two securing screws to that unit you mentioned were not very tight. Would that have caused the effects I was experiencing, do we think?

Anyhoo I took the top off the carb and removed the blanking plate to access the primary jet. Could not remove that jet, its tight. So I suppose that rules that out!
I wanted to check it was clean but hey ho.
I removed all the others I could see and all were clean. There was some sediment in the bowls again which is odd. I use only Esso Super petrol. I do have a filter which isn't very old, the glass type from ecas.

I decided to try and put a piece of tape over the feed hole for that unit mentioned by Roger V and when I started it -behold! Rorty and fun again. Relatively speaking.... I went around the block and it seemed normal again. I'm stuck at work until tomorrow but i will have the full road test then.
I didn't see any smoke at all.

Whats the smoke thing all about with that unit then? And could the not very tight screws have been the cause of my poor cold running and hesitation?

The car barely seems to use any oil, its still at the same level as when I last changed the oil, and its a nice colour.

I would like to say thanks to everyone for chiming in and helping me out. Its been a not very difficult learning curve.

And Roger V, you can be my favourite person temporarily

Sediment is in the petrol. A friend did let test the sediment at the technical university and it turned out to be iron/rust. And as the tested 2cv did have a plastic petrol tank it must have come from further down in the line...petrolstation, tanktruck, refinery...???