I received the MID unit today from Ireland, plugged it in and saw the set up screen for time and date etc.
pressed the code button and entered '0000' as the battery had been replaced.

she roared to life and all seems good with the world again.

can't thank you all enough for the speedy responses and advice.
the MID unit will stay in the car incase this happens again because we still do not understand why the code requirement came up in the first place.
When I purchased the battery 2 years previously the guys fitting it told me that this model does not require inputting the code after battery swap.

time will tell if it happens in the future.