Does sound like a dodgy button.
When stationary, the "info" button brings up the setup menu and "+" and "-" navigate it
When moving, the stalk-end button should cycle through all the data displays, starting with "Range". The "info" button should cancel it back to date and temp.
Should only need to hold the button for 2 or 3 secs to reset. It won't reset unless you've already got a data display showing (ie not the date / temp screen). If the screen you have showing mentions "Trip B" then the reset will only be the "Trip B" data.
Trip B, incidentally, is an additional trip logger that resets when the car has been "off" for 2 or 3 hours. But as mentioned above you can override that any time.
Which end of Derbyshire are you in? You can always bring it round to check it out if you need!