The lease may allow you to enter and inspect, enter and repair and enter and repair or re charge the cost of the works.
The lessee however can always sue for harassment and over breach of quiet enjoyment, and who wins/loses depends on the facts, and that while damage is being caused, the unit is empty, and it might be appropriate to simply tape a bag over it.
That said your insurance may require that properties that are empty over 30 days start to affect cover and combined with the gas you may have to have your solicitor send them a letter notifying that they are in breach and you intend to take action unless they remedy it forthwith.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers. More ramblings atleaseholdpropertymanager.blogspot.com