Update –
Cougar…cylinder was removed because thats what youtube told me to do! With hindsight i now realise that it was unnecessary although in my defence the deadlock appeared to be the problem when I first looked at problem, briefly!
There was no chance the keepers had moved – adjusting screws and holding screws all very firm. And I hear what you’re saying about not knowing if all lined up correctly but it just felt right when closed and handle lifted as if all locks engaged. When in this position the deadlock turned very slightly, just enough to prevent handle only opening the door, it definitely wasn’t secure enough.
So, I’ve just returned after a successful door lock fixing job……ensured dead lock lined up with keeper, it did. Sprayed some GT85 on deadlock, lifted handle ensuring the second ‘click’ could be heard (a mm or so extra upwards on the handle from its normal up position), hey presto, door locked….and unlocked….and locked…..and unlocked………but as far as my mum knows I’ve had door off hinges realigning it!!
Thanks for all your advice, I’ll know what to do next time if the keepers do eventually move, sorry it was such a basic remedy, not helpful to anybody really!