Is anyone familiar with this equipment named in the title?

I'm fairly new to the industry, just finished up my level 3 NVQ so i have never heard of these units before today. The building located in London has over 500 of these units installed in 1986 and relies on these operating in tandem with the perimeter LPHW heating, currently they are struggling to keep the office warm due to the Moduline units letting in to much cold supply air. It's to my understanding that the AHU's supply a fixed set temperature of air to each moduline unit, which then self regulates its own air flow based on the setting temperature for the thermostat.

I have found a few manuals on them but there's not much information on the correct calibration and how to check which, if any component is faulty. I have broken it down to the following components which regulate the air flow through the unit..
- control air filter (assumed to never have been changed)
- volume regulator (the commissioning lines are still in place, but since have all been adjusted)
- thermostat with bimetallic strip (i sense these may be causing a majority of the control issues)
- bellows

how would i go about checking the correct operation of the above components, when i adjust anything the air flow just seems to run wild and not act accordingly. Is there a certain procedure i can follow to go through and check each one individually?

I realise they are all probably well past the life expectancy and no longer efficient compared to todays new technologies available. But the client is insisting on trying to get it all working effectively, so if i can go round and list all the issues that need rectifying it may be the incentive the client needs to replace it all with a modern system.