However over the next 1000 miles or so the problem reappeared - I am not convinced that it was actually cured by the replacement transducer now.

As it was only occurring on very light throttle and especially with the Cruise Control set and  the engine was missing badly I was certain that it had to be the EGR system that was playing up. I also setup my laptop using the OBD2 Cable and set it monitoring the EGR Solenoid with an audio alert telling me when the EGR valve was opening - sure enough every time the EGR valve opened I was feeling the misfire.  I therefore started working my way through each component and it was whilst I was checking the EGR valve that I found the culprit.

The EGR Valve on the 24V Cosworth is mounted at the rear of the engine underneath the driver side inlet manifold and is difficult to see - I had waited for the engine to go cold (important!!) and then used my hands to feel around the valve - what I discovered was that the pipe that connects the exhaust manifold to the EGR valve was very loose. This resulted in an air leak allowing oxygen to enter the EGR valve which was sucked into the inlet manifold weakening the mixture. As the EGR only operated under light throttle that was when I felt the misfire.

The pipe uses plumbing style olives and unions and so the next job is to strip it all down and see what can be done. In the meantime I disconnected the electrical plug from the Pressure Transducer to disable the EGR function and the car was back to it's normal smooth self.

Update: I found a better way of disabling the EGR system by removing the pipe from the top of the EGR valve and plugging it. This seems better than disconnecting the transducer.

Test Method: By connecting a long pipe to the EGR valve you can suck on it to create a vacuum which opens the EGR valve. Doing this whilst the engine is running will detect any air leaks as the engine will stutter and the leak will be heard.

Read the details of the repair.