Baz1976, I am not qualified to give advice on Scots law, but if you were in England, I 'd say to pay the £90 to find out if anything is wrong. Like any claim, you need some kind of evidence that something is wrong with the car, though I accept that the low mileage itself is a pretty good clue!
If it does turn out to be a fault with the car, then I would expect them to sort it and refund the £90. If they don't turnanythingup, then I would suggest getting someone else to look at it, find the fault and put in a claim for the cost of the second opinion as well.
I read in another forum somewhere (though could be this one!) that someone had poor mpg and took it in and Citroen fiddled with the computer - whether that actually increased mpg or just made the numbers bigger on screen, who knows...
As an aside, have you tried manually calculating your mpg to double check the trip computer's figures?