FOUND IT!!!!!
Re tested the drain tubes, all fine so after an hour of ripping the plenum cover off and clearing all the debris that was stuck in there I put it all back together and confidently hosed it all over and washed it!! Opened the doors and hey presto, swimming pool still there, so i ripped off the a post covers, grab handles and sun visors, did my best contortionist impression and looked all round the windscreen seal as water trickled over the roof and bang, there it was!! The sealant along the top of the screen is beng breached by water, this is trickling along to the passenger a post and running down to the dash rail where it is filling up and lapping over into the cab on both sides!!
So...is it worth trying to seal from the inside (no chance on the outside cos of the windscreen trim) or is it a windscreen out and refit job??