You can tell them, make it part of the contract etc etc but try getting the message down to the folk at the sharp end when they inevitably carry an old battered and often untested 6 or 9 kg DP on the van.
Add often de-pressurised as they caught the handles briefly once a year or two ago (you don't need any powder to actually go down the hose for this to happen).
It is similar with construction/refurb works in general. It's funny how construction is recognised as being a high fire risk sector, but you often see the fire protection being either nothing or just a random pile of unserviced sometimes u/s junk acquired over the years from empty buildings.
Some of the bigger construction firms are better organised (e.g. Bowmer & Kirkland use Britannia P50's in boxes on trolleys with wireless fire alarms and the kit is always in mint condition) but a lot of refurb works are done by smaller concerns.