If the valve is a different size, it would probably mean a inflator would not fit?

Very good point. Would mean all garages with inflators having to fit different sizes for different valves/cars. Valves are just about the one component on a car where one size fits all brands (until TPMS came in but its only the hidden gubins that are different),

The OP needs to have a good look, its probably something very simple they are doing wrong.

You were 100% on the money. My local tyre company sorted it out when the car suffered yet another pozidrive screw injury to another tyre. The bottom line was that the valve that had been fitted was "a very cheap and badly manufactured piece of s***. Unbelievable that it was fitted in a main dealership who I am reliably informed do their own tyre fitting and don't farm it out. So everything back as it should be.