"A ‘Written Scheme of Examination’ should be drawn up for your pressure system and this should be carried out by a contractor acceptable to your insurers. The pressure vessel should then be subject to regular maintenance and periodic examination, by a competent person in accordance with the ‘Written Scheme of Examination’. Retain and file accordingly all relevant documentation."

What does this mean? I've found it as an overdue action point on an old audit, it's 18+months overdue but I don't know what it's really asking? 

We have our boilers checked monthly internally and serviced annually via a third party contractor who issues a gas safe certificate... It's asking for more but I don't really understand what it's asking for.

The 'written scheme of examination' is jargon from the pressure systems regulations.

First you need to check that this is indeed a pressure system as defined by the regulations; common examples are compressor reservoirs, steam systems, liquid nitrogen containers, gas bottles, autoclaves 'posh' coffee machines.

If it is a pressure system, as defined in the regs, then many people have their insurers draw up the written scheme (this was the case in the last two jobs I had). The sort of person required to do this is generally a chartered engineer with specific experience and competencies, as the written scheme defines what needs to be done and at what intervals in order to keep the thing in safe working order. Insurers also offer the inspection schedule that goes with the written scheme of examination.

Written schemes might specify things like witnessing the safety valves operate at a suitable pressure, internal examinations, etc. Intervals can be annual, or less frequent.

At my last job I had approximately 200 pressure systems, each with its own scheme. Joy! 72 of them were giant gas bottles re-purposed from submarines, fabricated circa 1932.

PS, for most people, gas bottles do not go on their equipment list because they are generally rented from the gas supplier. In these cases the gas supplier has to deal with the written schemes.