Now why didn't I think of that ....
Trouble is it sounds obvious but still doesn't quite answer the question
Quite a bit of force is required to get this gauge to drop down on both rails (it then doesn't exactly roll - bumping over the sleepers). As you can see it rests on one rail - by a smidgeon (guessed at 0.5mm) and when pressed down the rails are definitely pushed apart.
But the calipers show 32.02 between the inside faces of the outer gauge so that sort of confirms O at GOG current standards? When I finally open the kit I'll see what the "enclosed gauges" are, then possibly contemplate a O-MF set. Though building the kit with a wider gauge at the adjoining track and O-MF through the crossing sounds like adding one more thing to go wrong, two different gauges - neither of which will roll over the entire length - asking for trouble.
The reason the roller gauge bumps over the chairs is of course simply because it's too deep. With chaired track you need one that is just as deep as the rail head. More depth is useful for, say, soldered construction where it can assist in keeping the rail vertical. I don't know if this applies to Peco track, but chaired track along with some flatbottom is canted inwards at 1-20. I believe C&L holds it at the correct angle. This is another reason why you need gauges that are only head depth, otherwise they will try and pull the rail vertical as you press them down. This may be what is happening with the gauge on the Peco track, and appears that it is spreading the rails apart, which would be the end result.
Having used a non-standard b-t-b of 29.8mm in O in the past to gain finer flangeways, today, to me at least, the 31.5mm/O-MF standard seems the perfect answer, and esspecially if you are just starting out with new trackwork. Mixing standards dosen't seem a sensible idea, more a recipe for disaster.
Be careful though. Once you start making your own track you won't want to go back to using ready made stuff easily.