New locked gate

Access

Hi, one of our tenants is not paying her top up fee, we only get part of the rent paid by housing benefit. I have repeatedly gone to the house (and sent letters and left phone messages)but no one answers the door. I usually go down the passageway between our house and the one next door to gain access to the back of the house to see if I can catch anyone in the kitchen.Over a week ago a new locked gate appeared at the entrance to the passageway which means I can no longer access the back of the house. I have written explaining they did not have permission to put up a gate and requested a key. They still have not got in touch. Can I take down the gate? i know it's our tenant who put it up.