Doc,
I have disconnected the pipe from the EGR valve and plugged it (just like it was when I bought the van!) and it now does not stall!!
With it connected, moving the diaphram when the engine is warm and ticking over causes the engine speed to decrease. I could also feel the diaphram move when I revved the engine so I think it is working to some degree. The opening on the pipe from the EGR valve has a very small hole which I suspect may be blocked. I also had my doubts about the state of the valve as it was felt a bit rough when I was moving the diaphram manually! I suspect it needs replacing.
You were spot-on with your diagnostic! The engine coughed and spluttered when I moved off very slowly. It's fine with it disconnected!!
Thanks for the info on the ECM
Cheers
Disco