Many thanks for all the responses so far.
To clarify, the fitter had already tiled the floor including under the units. The 30ml granite worktops are due this week & our fitter was aware of this but our guess is he has not catered for them & when setting the unit height at 910mm. If it wasn’t for the height issue, we would accept trim being added to the plinths to cover the gap but unfortunately height is an issue, particularly for me. The worktop height would not be ergonomic & the working height for me at our gas hob would be unsafe.
it is a big job to lower the very long run of units as it includes sink & dishwasher plumbing. However, as lowering the island (which was to be long awaited my main pastry/pasta/cake making worktop) would be more costly because of replacing the glued sides & back panel, we feel we have to insist on the lowering of the other units.
Interestingly the fitter claims he set his levels from the separate corner run of units where we have a double oven & yet the height there is 15mm lower than all the others.
If only he’d admitted the error, rather than make so many ridiculous justifications, the situation would be less tense.