Decided to have one last go at sealing the Oil-pressure Sensor today, before I’m left to conclude that it’s the sensor itself which is the source of the leak and not the join between it and the engine block. I nipped off to Hellfords and scored a box of mixed size copper washers, in the hope that, amongst the range therein, would be some the right size for the job. Then it was off with the recalcitrant sensor again.
I discovered that, as I belatedly expected, the metal lip at the base of the screw thread, on the body of the sensor was actually an existing washer. It was a good tight fit –I had to ‘unscrew’ it using pliers, but it did look a bit rough round the edges: