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@Dammit
I knew this before, but apparently I'd forgotten it:
If you set my bike up according to the Shimano instructions then it doesn't work, as the b-tension setting required to put the upper jockey cage into the specified relationship with the largest sprocket means that when the mech is in the middle of the block the upper jockey wheel is too far behind the centreline of the cassette, and when it moves either up or down the block to initiate a shift, the chain won't follow as the mech doesn't develop enough chain angle to pull it over onto the teeth of the sprocket on either side of the one being changed from.
What is required is to set the b-tension screw so that the upper jockey plate strikes the largest sprocket when the bike is in the stand, as when your body weight is on the bike and the suspension is in its sagged position the upper jockey wheel plate will be in the correct relationship to the largest sprocket, and the upper jockey wheel will also be sufficiently far forwards in relation to the centre line of the sprocket that it can initiate and complete a gear change, both up and down.
It still clangs into gear on the smallest two sprockets, but the other ten gears are spot on.
My jockey wheel cage has "SGS" written on it, but the upper plate of the mech is marked in the same was as the 2x12 version, that has a maximum capacity of 45t, whereas my cassette is a 10-50.
^For reference in case anyone else every ends up in a situation where you need to add cable tension to shift up, but remove it to shift down - which I admit puzzled me for a while.