Thanks for the input, all interesting stuff. Thought I'd better post a quick update
Let me recap
Car regularly averaging 62-66. Went for new tyres and MOT, swapped original Michelin energy Savers for Goodyear Eff.Grip. As I got to tyre fitter, which is adjacent to main dealer, low fuel light came on. Tyres changed, round to dealer & MOT done, which included running car stationary for about an hour. Left for home and due to proximity of fuel stations was forced to put in 3/4 tank of supermarket fuel. MOT day +2 did a regular run to Taunton, on way back reset trip and struggled to get beyond 57mpg. Over following week of 'pottering about' and resetting trip several times consumption deteriorated to 52/53ish, not happy as discussed above, intending to run tank down and fill with my 'normal' fuel + go back to dealer for checks.
MOT day +12, still on same tank of fuel, had to go down to Plymouth (from just South of Exeter). As I dropped onto A38 I did something I've never done in this car, I left it in 3rd and floored it, almost red lining for 1/2 mile or so. Slowly, over the rest of the return journey mpg crept up and by the time I got home computer was registering just over 66mpg, roughly pre MOT/new tyre levels. Yesterday did another 'regular' run to Taunton, slightly slower than normal due to traffic, and averaged 69.4 over the journey (only according to computer, I know).
Now here's my theory!! Low fuel level plus running for an hour during MOT picked up some 'crud' from tank and gummed something up, then my 'blast' down the A38 cleared it out again?? Anyway, if this is not the cause, and it might not be as car seemed to be running smoothly, I'm still a happy chappie
Thanks for all your input. Dave D