Update time - I changed the harness and for a non mechanic, it was quite a simple job. Fired the Disco up and the misfire has gone. I would go as far to say that the engine has run better than it has for a long time. The harness was probably breaking down for a while without me noticing. The other thing I changed was the fuel. I was advised to move away from supermarket fuel, which I did and put good quality fuel in. It is a pleasure to drive again