Thanks for all the replies.

A code lock on the door would be more problematic for the relative,
and really this is about Social Services having access to the key.

I looked at CB's Secured by Design recommendation. It's tempting. It
may well be good. But I like it less after seeing the cheesy promo
videos and the OAP game. Our local crime prevention unit are
recommending the Toolstation ones.

So I'm going to go for one of the Toolstation ones or the Squire that
PP suggested, depending on what's available en route tomorrow.

Thanks again.