Getting a revalve
Mojo have checked out the shock and it is all working fine, so the weird action on the rear suspension was down to the very notchy DU bush.
They have however suggested that the shock tune should be slightly firmer.
They spotted that I was running 205psi (18mm/28% sag) which indicates that the L-Rebound L-Compression 175-Boost calibration was too light fro my not-so-svelte frame. This fits in with the feeling I was getting that the suspension could do with a bit more support in the mid-stroke for aggressively working the trail.
I'll be very interested to see how much the new setup retains the characteristics I'd been enjoying. The change I'm looking for is subtle because so many things were working right already.