having a dispute with another installer

[John.g]

Don't think It was a legal requirement back then either as it would have stated "must be fitted"

 

[Ric2013]

And if sealed system and prv after pump like it normally is

Is that correct, or just the rubbish you end up coming across? I'd be more inclined to want the PRV to be upstream of the EV and thus between the pump and the boiler, though I'm ready to be told I'm wrong.

Don't think It was a legal requirement back then either as it would have stated "must be fitted"

I think 'should be fitted' means 'and jolly well shall be fitted', though, being course material, it isn't 100% clear what was a legal requirement and what was just the author trying to influence his students. I note the only 'must's in that entire extract refer to the same thing: not blocking the path between the boiler and the open vent i.e. ON THE FLOW (even though the wording is clumsy and you need to take the two 'musts' together for it to be clear that this is what it means), so I think your interpretation that 'must' refers to a legal requirement is probably correct.