Neil - apols for delayed response. Yes I am sure. By some miracle, given the age of these doors and the extent to which they'd been 'butchered', I found a couple with the following fire cert label intact (See attached). I realise one has to be careful assuming that all doors in a residential block are identical but those that appeared original had certain aesthetic and constructional features that led me to believe they were from the same manufacturer. Furthermore, some doors did have a single 20mm seal so I'm reasonably certain this is a consequence of contractors, who are unfamiliar with fire doors, assuming they were FD30 rated.
I just wanted to ensure, before I report back to the housing authority concerned, that 15mm seals on FD60 doors wasn't some legacy standard.
Thank you as always.