Take it easy is my advice. First, just let them know what the issue is. If this is not possible in a casual 'Hello, by the way…' then just give them a simple note with copies of any relevant documents such as deeds attached. End the message with an invitation for a coffee and suggest some good times for you. If they are reasonable, they will make contact and all will be agreed. If they are unreasonable, point out what you intend to do, give them plenty of time and proceed as planned. In case of flaming dispute, get a solicitor to send them a letter explaining what you are doing and that it is legal. Good luck.