After serious thinking Dave Plowman (News) wrote :
A fixing into a plastered brick wall through tiles.

Most of the fixings went well - wall plugs pushed through the tiles into
the brick. But one obviously hit the soft old mortar, and won't tighten.

Anything that could be squirted through the hole to give a decent fixing?
Or any other ideas? I'm already at the limit of the longest screws I can
find in that diameter.

Apply some grease to the thread of 'your longest screw', then mix some
Plastic Padding car body filler and quickly before it sets and using a
syringe, squirt it deeply into the hole. Push it in as full as you can
get it, then put the screw in, but not all the way. Later, once it has
set, you can tighten the greased screw.