I had that problem too and solved it by removing the membrane ans soaking it for a few days in plain gasoline. If it has been sitting for some time, then the membrane tends to stiffen up and will not effectively pull the fuel to the carb. Also, when you pul the carb, be sure to blow out all of the air holes and likes just to be sure there is nothing restricting the air flow. One other possibility is be sure to check that the gasket is mounted correctly, On the older ZDZs, it was easy to mount the gasket so that the vent hole was blocked.
Hope this helps!
PS. one more thing- if all else fails, you might either have to get a new carb or better- a rebuild kit, that's a bit cheaper. Hopefully you have a good ZDZ dealer in the UK. If not, I can recommend one from Holland. He's very helpful in matters like these.
(email address- firstname.lastname@example.org
ask for Johann. His website is all in Dutch but he speaks and writes English.