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Hello, this is Jim and welcome back to JustAnswer.
Thank you for the question. The lease terms are likely to say that pets are not permitted - it is a usual clause in a leasehold property given the fact animals can cause damage, the buildings insurance could be void, hygiene reasons, excessive noise, etc. Sometimes freeholders can allow a pet if there is a good reason, such as an owner needs one for medical reasons. The dog you mention should be leashed as the communal area is not private property - if the dog were to bite someone, the owner would be liable under the dog legislation and the Police could also become involved (the dog could be destroyed and the owner prosecuted). So there is a risk there if the dog is off a lead. It is entirely unfair that the dog entered your property too. The owner could face forfeiture proceedings due to breach of the lease - meaning they would face losing their property. The terms of the lease would need to be checked but even if they did allow for pets, the fact the pet was off a lead / entered your property is unacceptable too. A lawyer's letter could be the way forwards to deal with this - and forfeiture could be applied for, to the First Tier (Property) Tribunal which is a risk for the owner.
I hope this answers the question. If you have any follow up questions then please do let me know.
Just a final note that I am free most days and would be happy to assist with any other queries you may have.