...can he legally do this & extend the date without issuing a NEW section 21?
The Section 21 notice you received will have had a date on it.
Along with that date will very likely have been the word "
after".
There's your clue. You are in receipt of a genuine goodwill gesture... and, ideally, you need to be preparing to move out. The 11th February is only 3 days away and you're on here asking about legalities. So I guess that you are now contemplating asking for another goodwill gesture, or maybe commencing a few shenanigans? Either way I'd probably open up a discussion with the Landlord. Usually best for all parties.