To clarify on the "not that accessible" point, they are pretty obvious features of the room... one is right in the middle! What I meant is that it's not a space we regularly use, it's effectively a cellar so we simply didn't go in there for some time after moving in.
When we looked round there was some stuff in there but it was by no means full to the ceiling. We didn't notice the problem so probably a box was in front of the base of that pillar, but the moment I noticed there was some rust I spent 30s looking at all 3 and it was very obviously a problem. That is surely within the scope of what a surveyor should do.
In terms of "they weren't exactly incorrect to say it is corroded", well sure. But this was labelled on the survey report in the same severity as about 100 other things including "the wooden windows need painting periodically".
Crucially, it did not say "this should be inspected more thoroughly" which is what the survey says for anything they deem even slightly at risk, it's their go-to cop-out clause in case something more serious is hidden
It seems there is a variety of opinion but putting that to one side, IF I do feel aggrieved should I start by simply emailing my contact to raise it, or should I go straight in via a solicitor, or something in-between? I'm not litigious by nature but the fact is I would not have paid what I did knowing this problem existed. If they had even said "you need to have this checked out" I would've been forewarned.