i just worked out, when it does play up the fuel level in the bowl of the carb does not drop. does that mean it is not a fuel supply problem?

Since you say that you have had the carb. rebuilt, I'm guessing that it is not a fuel problem.
It sounds like you may have an electrical fault in the ignition primary circuit.
[sounds like your secondary/high tension side has been rebuilt]
Have a good look at the wires going to/from your ballast resistor and even the resistor itself.
Wriggle everthing while the engine is idling and see if there is a miss/cutout.
Can you describe what happens when it goes wrong, engine dead, still running, no guts or what?
Aways difficult to diagnose from a distance, but ....

when it plays up it has no power and sometimes stalls if at idle. pulled the plugs out just after it played up and they were light brown, indicating a lean mixture?? they are normally a bit black when i check them.

Tan is good, ideal mixture.
I still think it is electrickery

Have you got the anti-diesel solenoid hooked up?

is that the little soleniod on the front corner of the carby nearest to the rocker cover, if so yes it is hooked up.

Sure is, to test it put the ignition on and unplug then plug it back together, should make a click sound if working ok.

it is working. i had that disconnected once before and the car wouldn't idle. the car is going to a mechanic tomorrow, so hopefully he will fix it asap

IT'S ALIVE. turns out i was the carby. 28/36 weber going on hopefully on the weekend. Thankyou everyone very much for your help.