UPDATE
It was code P0155. This is heated oxygen sensor (HO2S) 1, bank 2, heater control circuit malfunction. I assume it is the oxygen sensor on the drivers bank of cylinders at the engine end, near the exhaust manifold.
Is it easy to replace?
How much?
I cleared the code, and also checked the petrol cap. It seemed ok, but I cleaned it and put a smear of Vaseline on the rubber seal and metal pipe to make sure it was air tight.
As I had bought some MAF cleaner, I have also given this a clean.
Some suggest it may be a faulty fuse. To be checked. Their are no systems such as rough running or starting. It is also returning 32 mpg on a long run.