Hi Wiz, no need now mate, thanks. Yendor has explained the problem, & his suggested solution having looked at a wiring diagram of my set up is to try adding a further earth cable between the neg terminal of the crank battery & the chassis. I am optimistic that this will fix the problem. I had not considered a) that the chassis itself is not earthed directly on Patrols ( had tested continuity, but this doesn't tell capacity, only connection & b) that it is possible to overload an earth. If this solution doesn't work for me, then the problem will most likely lay within the instrument cluster, but as I say, I'm optimistic that this will fix the issue.

Others have previously said 'earthing problem' but I hadn't really understood just what that might mean until Yendor,s post.
Anyone else with the problem should try the simple adding an extra earth direct to chassis. I think your additional earth would have achieved the same outcome, but easier to run the earth from crank battery to chassis, rather than, in my case, another wire from inside the pod to under the bonnet.

Cuppa