ArthurR,
Although many military aircraft are on an ATA-type of Chapter system you will find that most UK military aircraft, especially older types, have different ATA chapters for similar systems, e.g. Hydraulics on one type might be Chapter 25 but on another it may be Chapter 65.
It would be nice to think that after all this time someone in an MOD building might be trying to standardise things - but its only taken 90+ years to get to this "embryonic stage of military aircraft maintenance".