Fixed! Was the very pipe we suspected. It was gushing out at the join between the metal return from the pinion valve and the rubber that had gone quite hardened. Funnily enough it wasn't actually split - the clip (looks like a factory fit) ha possiby just gone weak. The LHM was running out in a steady drip but there was one needle thin jet aiming directly up the back of the engine that landed on top o the RH strut top causing an initial red herring 'strut top it leaking'! '...but suspension staying up... ah, not strut top'!
So the 8mm hose was just right and I used the petrol pipe type of hose clips as they're so much better at holding smaller diameter hose. It was a struggle to route the new pipe through all the guides from the LHM tank to the pinion valve (especially in the unrelenting rain this morning) but I got there and I've tripple checked it on several urban and motorway runs from noon till half 3 and all seems good. Also, technically just changed the LHM
Thanks again all!