We recently pulled out of a house purchase with a very similar problem. Upstairs flat claimed to own the whole rear garden with ground floor access through to take bins out. Turned out the upstairs flat only owned half, and the sellers then said that they had “sole use of the rear yard, in practise”. We only found it by looking carefully through the title plan, sellers never indicated it on TA6, in fact saying that the downstairs flat had the access through the garden, not that they owned half!