Hi George,
Thanks for the advice. I checked all the hoses I could see, both in the engine bay and under the dash, and they looked ok. I pulled off the valve between the manifold and the vac reservoir in the engine bay and sucked on it and could almost convince myself it had a leak, but it didn't seem very significant.

I also pulled off the small hose at the other end of the vac reservoir (the one that goes into the car) and sucked on it. I kept sucking, and air kept coming, so I suppose it is inside somewhere. However, when I blew, it kept pressure (to the extent that my lungs can make much pressure), which left me a tad puzzled.

So I'll try and get a valve for in the engine bay and try that, mainly because it is a hell of a lot easier than trouble shooting everything under that awfully crowded dashboard!

If you have any other suggestions, I'd love to try them.

Thanks

John