Hi Guys
Have found the cause of the fluctuating pressure. It is indeed a faulty earth. There is a short earth which connects the body to the chassis directly below the oil pressure sensor. When broken, the oil pressure fluctuates wildly. Incorrect installation is the cause of failure ( with the lead pointing down instead of up to make an easy connection with the sump) . This puts tension on the lead causing it to fracture probably after several large potholes or a starter exchange. (Jacking the body to give better access to the starter could also break the earth connection ). Removing and re-installing the earth taking care to replace correctly will fix the problem. Will supply pics if requested.
Thanks and happy motoring.