Another update...
Tonight the tank was showing 1/4 full on the gauge. I filled up in the garage and it only took 26 litres to fill it. That means I have used only 19 litres to this tank so far. I had completed 160 miles.
It must be a problem with the level indicator. Before I filled up it was showing 52 miles to empty but yet it would appear that it still had 19 litres in the tanks. MPG readout is still in and around 30mpg.
She is due in for an MOT check very soon. What should I be asking them to check?