The tenant in the ground floor flat cannot deny his neighbour upstairs access when it is required for purposes such as cleaning his windows.
The neighbour upstairs, however, would do well just to politely advise him when he wished to do so, and give him a day's notice.
This reminds me of an upper floor flat which I used to clean. The lady downstairs (retired spinster librarian) hated the idea of giving access to me, and made it nigh on impossible by planting shrubs and leaving plant pots all over the place. She would go off on one if I so much as touched anything. She would stand and watch my every move.
I put up with this for a few months, but eventually told my customer that I couldn't work this way anymore and stopped doing them.
Later, while doing another flat opposite, I saw Miss Librarian fall over one of her own pots! I know we shouldn't rejoice over unrighteousness, but I did enjoy that one!
John