I am looking for information as to what height I should put the 'Fire Door Keep Shut' signage on my new fire doors. There does not seem to be a statutory height that I can find in the Building Regulations.

There is nothing in building regulations but you will find the following in BS 5499 part 4 2013,

The mounting height on escape route signs should be sited conspicuously within the normal field of vision and to assist evacuees with the prediction of the location of successive signs.

1. Signs above doors or open spaces should be mounted between 2 m and 2.5 m from floor level, measured to the base of the sign and be sited as close to the centre line of the escape routes practicable; this mounting height is intended to ensure that signs can be readily seen, e.g. over the heads of people.