Gap between door casing and architrave - how to fill?

should work. Or you can use about anything to to the mass fill - polyfilla is good

If there is likely to be movement use decorators caulk for the top finish

I would have removed the architrave, extended the door casings into the room using some strips of wood the same thickness as the boards, and then boarded up to the strips and re-fitted the architrave to cover the edge of the board.

I have the same problem and intending to fill the gap with PAR timber flush with the door casing and refit with a new architrave.

Do you mean a gap between the back of the architrave and the wall, or between the architrave and the door frame because the plasterboard now stands proud of the door lining?

If the latter:

You've made me think... The door casing is only held in with foam and has no architrave on the other side under the stairs, i'll pull it out and refit it an inch forward.

"and want to be set back from the edge of the lining by 5 to 10mm depending on preference"

Now why is that, i've gone to pains to make them flush! (To make any potential filling easier, except as I think about it it wouldn't really have made any difference..

Like most of these building "rules", because it was decided at some point that it looks better... however there is usually a practical element as well - if you fix things flush, then you can normally guarantee they won't remain so with seasonal wood movement - so better to make a feature of the reveal rather than have a line that splits the paint each season.

Apparently having some sort of gap or inwards step in the plasterboard around door linings is the 'modern' way -

Interesting. And my hinges won't go back the same way without some chipping away.

Now there's ugly for you :-)

(not to mention built in dust traps, with lots of places for shrinkage and settlement cracks to become visible)

Indeed... and that is one of the reasons these things are traditionally done, so you can fit square and plumb lining to what might not be a perfectly matching hole in the wall, and then "tune"[1] it to the door to get even looking gaps all round. Ultimately you need a way of hiding the mechanisms used to make this all work from view.

Hence on reason why you have architraves to hide any gaps, packers, and wedges.