Try the other carb, if the problem is still there then....

Update:

I took out again the idle and mixture jet/needle, sprayed carb cleaner down the orifices, and changed the mixture adjuster for another. Should the tip be flat or pointed? The three I have are all non-pointed at the tip.

Anyway, the car now 'seems' to run slightly smoother.

I have strobed the timing, ok. Checked the tappits, one exhaust gap slightly large.

I sprayed as suggested round the manifold and carb base. The engine did slow down and I heard a sucking noise, but I am pretty sure it was excessive spray being sucked through the tiny hole under the manifold.

I have spent enough time under the bonnet getting back-ache so I shall trundle off across the pond in a few days and resume with the problem on my return: the little SOB is still slowing down and stopping on hard braking, so I won't brake hard!

With the standard of driving here in France I think you might find yourself needing to brake hard 7

Idle mixture screw should be "pointy" - if they are very flat, they have been wound in too hard in the past.....

Is this pointy enough?

How about the plug colour, 2 sides of the same new plug. Both plugs the same.

Thanks again.

how much running have those plugs had? minutes or hours?
Possibly on the lean side - what r-are you doing when replacing the idle mixture screw just winding it in the same amount or setting it.
https://www.youtube.com/watch?v=XOxxS1EMphU
https://www.youtube.com/watch?v=KWdZSr7x81w

Thanks for the links Sean, nice basic info.

re: the plugs. the pix were taken after only about 8 miles driving in traffic. I've just returned from a fairly fast 40 mile trip and they are the same colour.

The engine does spit through the carb a couple of times in the first 30 seconds starting from cold with minimum choke, then is fine.

Regarding setting the mixture I am setting the tick-over fairly slow and turning the screw until the engine starts to get a little lumpy and slows. Then turn back one turn and re-adjust the tick-over.

That's about right isn't it?

I really appreciate all this help: you've all been round the block more than once and rather than continue to smack my head against the proverbial brick wall, I bow to advice from those who have tried, and done.

Mixture screw only adjusts idle, not running mixture - check the size of the main jet (in the right hand chamber) this should be, according to the maestro Ken, 107.5 for modern fuels......

Im convinced its way too lean check the manifold to hed joints for leaks too

Well, in spite of my slight concern about taking the car to France, we've just returned from a trip to Normandy with no problems, covering 700 miles.
Yes the engine did come to a halt under hardish braking and yes the engine was still running slightly lumpy on light throttle.

I've posted pix of the plugs: After such mileage and fairly fast driving, what do people think of the colour.

Too weak? Just right? All opinions appreciated.

Thanks for your input.