The round bit is a shroud. It will either pull down if you put a scraper or something in. Might be stuck up with putty or it will have a thread on it you should be able to turn with your hand.
Use a bit dpc or heavy visqueen for a new washer and punch a hole through it with a bit 10mm pipe rather than trying to cut it
All done, the round nut was a shroud... There wasn't a membrane as such, like a big metal type disc in there that had corroded & fell apart... Luckily my membrane material worked okay, fingers crossed it holds lol!