I have a paper copy of this pretty antiquated standard.
You dont say exactly what you are after but Standard says

3.8.1 "pipe insulation shall have an R-value of not less than 0.3 m2.K/W or conductivity (K) of not more than 0.04 W/m.K."

3.8.2 The following are acceptable materials for the thermal insulation of hot water pipes:

(a) Closed cell foam polymer insulation 12mm nominal thickness
(b) Fibreglass insulation with a nominal thickness of 12mm which is preformed to the shape of the pipe

3.8.3 Any insulation material that absorbs moisture shall be protected by a waterproof coating or cladding when insatlled in damp or exposed conditions. Any insulation material affected by weather or sunlight must be protected.

3.8.4 Materials used must be capable of withstanding the temperatures generated by the system.


With a quick flick through it I cant find any reference to 25mm but might have missed it.

Cheers
Grant