My glow plug lamp is continually flashing but when attached to the analyser no fault is shown. I have had the plus changed and still it flashes. Other symptoms I have are bad starting 10 - 15 turnovers before it starts and then it runs rough with clouds of white smoke and a rattle in the engine. After a short while the engine settles down and I can pull away. The smoke then soon stops and after about a mile the engione runs normally. I am still getting 55ish mpg and the performance does not appear to be affected.

There is something seriously wrong with your car.

I had a 2.0 TDCi Mondeo and on average got approx. 40 mpg. On a holiday run could just about squeeze 50 mpg out if it. If you are getting 55 mpg you own a one off car and I would recommend that you do not touch it in any way.