I was on a flight about to land at LHR, when an elderly man went to the WC.
He was in there for about 5mins, by which point we were on final approach.

It was only when the cabin crew went to sit in his jump seat adjacent to the WC he noticed the WC was still locked.

He knocked on the door, but due to the language barrier he didn't get much of a response. The crew then used the phone above the jump seat, and informed either the CSD or flight deck (unsure which) the 'cabin was not secure'.

We still maintained our approach and another crew member appeared to force the door open and remove the elderly man. He was returned to his seat and we landed a minute or so later.