SLS has given up and Disco is on its bump stops at the rear. Dealer thinks it is "probably" the valve block (tested and SLABS ECU showing no faults). Famous Four will do me a new valve block for GBP132, and apparently it's an easy fit. On the other hand, I can get a coil spring conversion kit from several places for GBP70-100, although it seems that I will need to visit someone with the right kit to fool the ECU afterwards.
Watcher think, chaps? Bear in mind I like the SLS and am a big fan - when it works. :-(