Must admit I've had a bit of a revelation over the last couple of rides, riding my CXer on some of my normal mtb trails I find that the CX bike is not only faster but *gasp* actually more fun, at the expense of some comfort of course.

What really surprised me was how much better the cx was at carving turns (on loamy woodland singletrack) – it seemed to have so much more grip and I could lean in to the turn far more even on the cheapo 35c Schwalbe land cruiser tyres compared to sticky rubber nobbly-ish fatish mtb tyres. So forum – why is this, is it purely a function of wheel size?

Got to be wheel size, I ran Landcruisers as well and I could out run my mates MTB's in the woods. And like 29ers they laugh in the face of cambers.
You start to notice the turn of speed an MTB has in the very tight stuff, have you ridden your crosser with others on MTB's?
Crossers as I've always said have their moments.

Long, thin contact patch vs short, wide?

I must admit whenever I switch back to the cross at first I find it a bit sketchy but speed up quickly once I get used to the different feel. In mushy stuff once feeling confident the wheels seem to sort of dig in and you can predicably drift both wheels on corners whilst the bigger mtb tyre stays closer to the surface and is either gripping or giving way with little notice. Thats my perception in any case.

Longer effective stem? (assuming you're riding on the hoods or drops them you effectively have a very long stem compared to an mtb when you consider where you're actually holding the bars.

A longer stem is more stable (barge-like!) which is bad for fast twisty stuff but actually good for sweeping bends where you don't want it to be twitchy.

most likely a combination of different geometry and where your weight is distributed.

more weight over the front of the bike giving the tyre better bite?

I've recently been experimenting on my rides at really pushing my weight forward into turns & it feels like I have more grip doing this.
How much of this is actually due to better grip & how much is down to me believing there is more grip I don't know…..but if it works I don't particularly care.