[DanBC]

> I need to be aware of any name changes of him

You don't.

You have a contract with the person currently calling themselves John Doe. When they call themselves Jo Doe and then Jack Bob and then etc it's still the same person.

Currently in England you're allowed to call youself what you want, so long as you don't have the intention to deceive. In practice, 'deed poll' or marriage certificate are handy if you want people such as banks or doctors or airlines to actually use your new name.