Landlord took lock out of door without permission, now wants payment for damage!!

Hump

The important bit here is that the landlord has taken a deposit and presumably not protected it, meaning that his s21 based claim for possession is going nowhere fast.

So the landlord is now applying juvenile pressure to get rid of your godparents! Harassment? - yep, anything anyone is really going to do about it? - nope.

So the best strategy is to - as others have said - change the lock barrel and refuse to give the landlord a copy of the key (and replace the old lock upon leaving). Keep a record of the landlord's action (time, date and what he did) and wait ...... if there's any repeat, harassment or any other strange act by the landlord then I would suggest the following:

1. Attempt to contact the local council's Tenancy Relations Officer (if they have one) - you could ask at the environmental health or housing advice departments
2. Send the landlord a letter - outlining the acts of harassment and demanding that the landlord should desist from any further contact apart from in writing - any further harassment or action and you will consider lodging a claim under s27 of the Housing Act 1988 for damages in the local county court - keep a copy of this letter and a 'proof of posting' free from the post office

If the harassment continues then consult a solicitor about bring a claim under s27 for damages - the 'warning' letter will demonstrate to the court that you tried to resolve the issue before bothering the court.