My system has been empty - various leaks - for more than eighteen months since I bought the car. I recall reading a post sometime from someone who claimed to be a specialist, saying that a system that had been empty needed more than just refilling because of also being open to the atmosphere ( pollution perhaps ? ).
Does anyone with experience agree with that, and what would this extra work/procedure be called ?
Yes, that's correct. The most common refrigerant is R134a and the lubricating oil it uses reacts with water to form quite nasty acids which can corrode any steel parts in the system. As a result, there's a "receiver/ dryer" in most (I think maybe all?) systems. This is a filter, but also contains desiccant granules to absorb any tiny amounts of moisture that may find their way into the system. If the system has been open to atmosphere at any point, moisture will have found its way in and the granules will have been saturated. If there's any refrigerant pressure in it at all, then you might get away wit it, but if you poke either of the valves in the charging ports for a fraction of a second and nothing tries to escape, there will almost certainly be moisture in the system.
I've worked on a few aircon systems in the past, and the most common failures seem to be the condenser. It's right at the front of the car and is prone to stone chips. After that, it's hoses. (and the sealing of joints between hoses and hardware). If you want to replace a condenser/ receiver/dryer yourself, they're readily available and not that expensive:
https://www.autoairconparts.co.uk/
HOWEVER, if you expose a receiver / dryer to air for more than 20 minutes or so, you can saturate the granules so it's best to leave that until last, and then quickly evacuate the system. The problem is that if you find a leak, you don't have long before you've ruined your new receiver / dryer. For this reason, it's usually best to leave it to a pro, who will have all the necessary equipment. I'd tend to steer clear of Kwik Fit and the like. They use automated charging stations whereby the operative just enters the make and model of the car, and connects the hoses. The charging station does everything else - evacuation, vacuum test, pressure test, adds the right amount of refrigerant and oil, etc. They're fine if it just needs a bit of a top-up but not so good if the system actually needs parts replacing.