Is a flat notional fire door of 42mm compliant ? Is there leeway from the 44mm 'rule' ?

Possibly yes. It will depend on the door's construction, age, condition, and any available evidence of fire performance. Do you have evidence that the door has successfully achieved a 30‑minute fire resistance test or has certification to support an FD30 rating?

 

If not, and you are relying on the door being a "notional FD30" fire door, a detailed assessment by a competent fire door inspector would be required. Thickness alone does not determine compliance, although a 42 mm door will require closer investigation than a typical 44 mm notional FD30 door.