Thanks to Joel and DesertRat.

As you say, this is a cheap(ish!) item, so itīs certainly worth checking, but I suspect it may be more complicated because of the repeatability of this problem. It ALWAYS happens about 10 minutes after starting - just about the time that something or other would be adjusting the mixture/timing or whatever, I suspect.

Another factor I have discovered is that, if I drive the car every day, it doesnīt seem to happen - only if itīs been left for a coulpe of days or more. Even more puzzling!

Still, I didnīt believe that resoldering the joints on my cruise control would fix it, but it did!

Cheers,

Phil