The oil level warning light has come on, saying that the oil level is too high. It was serviced a couple of months ago and no oil has been added. Is this an electronic glitch or is there another reason ? (have already thought head gasket and rapidly thought about something else!)

they are very sensitive to oil level, if its on the max its probably too high

it is unlikely the garage will accept any faults as its won due to the time differential etc.
hav u checked it manually say on the dipstick?
Look at the oil ,is it milky?
if? the level is increasing it can only be due to two things, oil is being added! or water is being added! ( gasket leak)
failing that the last & most likely expansion is..( after checking levels manually of course) the sensor itself has failed thus showing an erroneous high oil level light. it may be it has been set to default to this when it fails. check the level with the dipstick, then if? the level is correct & the oil is not 'milky etc' have the sensor checked out.