Dear Sir,

It would seem you've appropriated yourself of part of the communal garden building your own terrace.

Your other neighbours are entitled to use that part of the garden as its a communal area, not privative, belonging to all.

If they agree to use that area and leave evrything tidy that would be great for all involved avoiding further hassle and fuss. But it will be hard to enforce in practice whilst you are away.

Yours faithfully,