Thanks - that has really (I think!) helped clear something up in my mind.
Take this scenario, goal kick (or free kick, say) is being taken by reds. Red centre forward is on half way line, a pace behind him is blue centre back, marking him, as they both wait for the ball to drop.
Bit of movement from both players, red forward takes half a pace back. Blue defender gently pushes red forward in back - more a natural reaction to the closing of personal space (the sort of thing you might do in a crowd)
It is a push, but not careless, reckless or with excessive force, so no foul?
I've discussed this with a very experienced ref who says as soon as the hands go on a player in this scenario you should blow for a free kick - either against blue for the push or against red for backing into his opponent (but what offence is this? Impedes the progress of an opponent (and hence IDFK)?)