I know that the clearance on a fire door has to be 4mm (vertical edges and top of door?) and 8mm bottom of door BUT is that 4mm or 8mm measured from the outeredge of the door itself OR the outer edge of the intumescent / brush strip. I cannot find the answer to this anywhere!!

 

The clearance gap is between the edge of the door leaf and the door frame reveal. It should be 2mm to 4mm and the smoke brush or blade should fill the gap but without causing binding as the door self-closes.

The threshold gap (at the bottom of the door) should be either less than 3mm in height or fitted with a threshold seal.

See here for more information.

is the gap a the bottom of the door measured on the outside or inside of the door

The max gap at any point, anywhere along and each side of the threshold for fire resistance performance.

However, when measuring 3mm for restricted smoke spread the gap at the door bottom edge should be no greater along either side. Refer to BS 8214: 2016 Code of practice - Timber-based fire door assemblies for more information.