On Thu, 20 Oct 2016 21:13:48 +0100, Michael Chare
wrote:

snip

The thing is ... none of this has been touched and until this recent
incident, has been working 'just fine', especially the spilt spindle
thing?

That's not to say it not might all appreciate a drop of oil or smear
of grease etc but I feel it's more to do with the deadlock than
anything else. I guess the spindle could have an impact if it stopped
the handle lifting the shoot bolts high enough to allow the cylinder
cam to turn, but it does seem to work ok for the first lock position,
just not the double?

snip

I would try raising the handle with the door open just in case the shoot
bolts are not entering the frame properly.

Update:

I replaced the lock with a new one and now it's all working
(specifically double-locking) again.

However, to be able to undo the door from the outside you have to pull
the door hard shut to be able to turn the key (it only turns a few
degrees anyway) and release the latch?

From the inside it's easy to undo the door because the handle releases
the latch (greater mechanical advantage).

So, I'm not sure if something still needs adjusting or it just needs
to wear in a bit?

So, should the door be held tightly closed by the shoot-bolts to take
the load off the latch? If the shoot-bolts and their plates are worn,
allowing the load to fall onto the latch, could that be a cause of
this issue?

I am reluctant to ease any of the components in case it's simply a
matter of it all bedding in and I don't think the seals are the
problem as they still feel pretty flexible?

Cheers, T i m