ok, so final line on this one. Chris at Dynatune plugged in standalone sensor / gauge and we got 25psi at a hot idle with what seemed like pretty thin oil. Took some pics of the oil pressure sensor and had it confirmed it was the wrong sensor. How on gawds green earth the wrong sensor was in has not been established but that explained the crappy pressure reading. Happy I actually had oil pressure and it wasn't leaking either, I took the car up over the Applecross and enjoyed driving it about the Highlands. Got a replacement sensor sent direct to me and Chris fitted it last week. Fresh oil and Nissan filter fitted yesterday and lo we now have cold cranking pressure and of course the all important hot pressure at idle. So for a dodgy sensor way back in January, 2 senders and a replacement mid sump, several months of pillar and post shuffling and the best part of a grand later, I can breathe that the job is done. Seriously wtf!