Hmmm..thanks all for your ideas..I still have not got it sorted, and it is driving me nuts now.
but I shall overcome etc...Water in the fuel is a possibility, quite a good one, will try something to combat that.
I do have the correct spacer twixt pump and block, but not between carbs and manifold..however I don't think it's that, because all has been fine before without them. (yes, it was a b20a, now a b20 whatever with big valve head and two carbs, two door car) Two days ago it got so bad it was ridiculous..drove perfectly and pulled like a train for half an hour, and LITERALLY as I said to myself, well, I seem to have sorted that out (after swapping pumps again etc), car lurched like a loon and gave me a great big volvo finger. I got home, just, left it for 10 minutes and ventured out again to look at a mk1 golf gti, which wasn't..anyway..car was undriveable, must have died 15 times. Each time it would restart after a minute, on the button, then drive for progressively less and less distance. The longer I leave it the further it will drive. Swapped thoughts onto ignition, ie coil getting hot, but still the same after 3 coils, new leads, condensors, etc yawn. I can stall it with the accelerator, like a switch, in that if I give it a little bit more throttle than usual (which is not much, doesn't need it), car will instantly die, as if I am either flooding the spark or not getting fuel, but I suspected the former because the reaction is so instant that fuel in the float bowls should in theory prevent the latter. Hence my ignition circuit swap about. Car does not seem to be getting any hotter than usual.
Any road up, I'll leave it there. Told you it was driving me nuts :)