Gaining Entry of property due to overdue CP12

Maintenance

Hello,
I have a tenant who we are in the process of applying for a section 21 for the second time after the first got dropped due to a technicality. However the CP12 is now two months overdue (we thought they would have been out before then) and we can't get into the property as they have changed the locks.
I have sent them two letters with regards coming round to service the boiler however they never reply and their mobile phone has been disconnected so there is no way to communicate with them.
Is there a procedure that I need to adhere to with regards entering the property to get my plumber to service the boiler? and can I also change the locks? and if they aren't in the house do I give them a key as they haven't paid the rent for months?
Any help would be greatly appreciated...
I look forward to hearing from you

Kind regards

Mark Clayton
07975 729255