If you own the flat, check the deeds to see whether its a shared garden - if you rent, contact the Agent or the Landlord.

We own flat and its highlighted on plans and stated ln deed we have 1/6 of garden/drying green.

Then you need to address the issue with the person who lives downstairs who's having the work done. Otherwise, its the legal route - see a solicitor, and you may end up doing that anyway, if the person downstairs is uncooperative.

Some solicitors will give free advice over the phone, just charging if they get involved eg writing a letter, check local yellow pages

How are the other flats affected can they still get access? If they car'nt then get together and decide what to do as a group but do try to avoid the legal route as it can be costly.

Some people can be really obnoxious!! Presumably if he/she put the fence up and no-one complained, after a while they could state the garden was theirs alone, knowing that it would involve lengthy time in court and the lawyers fees to prove otherwise. Nip this in the bud now, before it gets out of hand. Try to be friendly first and find out if they know that access is allowed to everyone else, it could be they think the former owner just allowed other people into their own garden and they have no legal right. This is not the time to be shy, speak to your other neighbours, even if you never have before, and ask them what they think about it. Best of luck, but don't give up this right to an outside space.