"Anyway ... what do you propose in cases where Manufacturer A recommends (or should that be "mandates") a six-monthly I/PM cycle, whilst Manufacturer C (of similar kit let's say, sitting alongside in the same department) says that once in three years is enough."
A fairly extreme example, but nonetheless, examine the risk, decide to harmonise I/PM cycles if possible (which may include decreasing or increasing frequency) propose changes, then get them signed off by a representative section of interested parties.
(In reality, if one manufacturer recommended a six month interval and another manufacturer of similar kit recommended a three year interval, after looking into the validity of the claims, I would be looking standardise on the more reliable kit that required less maintenance).
You may deviate from the technical authorities (manufacturers) recommendations, either increase of decrease the frequency, based on pertinent factors.
So long as you have completed a fully documented, and approved risk assessment, the main point being to involve interested parties from the outset.
