Look, mon petit,
this "flat" deal is not gonna happen... Or, it may happen , if you play your cards right.
If is NOT meant to happen, SO BE IT. Don't get ill over it, move on, OK ?
We've had a similar experience when "we" (my husband and I)bought our ground floor maisonette in North London, near Barnet. At the end we got fed up with the 'red tape' and said:
" This is what we want , take it or leave it !"
We got the WHOLE of the front garden & very cheaply, too!
This is VERY un-usual/rare in a maisonette with normally a portion of the garden at the back and SHARED garden at the front.
We are the envy of all the neighbourhood, now, because of our tough stance in negotiation.
BUONA FORTUNA ~ BONNE CHANCE ~ GOOD LUCK !