Very long story short!! (telephone) call to Thetford spoke with a chap who was confounded by the fact that there was no drain out let, and the fact that despite trying to fill up via the filler tube it did not reach the holding tank.
Eventually he did ask if I was using the filler extension which it would appear i would need as there is a water pressure issue (the water needs to be under some pressure to travel to the holding tank ie up hill)I have seen this in the schematics but never got one with the van.
I will now order one and see if that works
Little piece of £2.63 plastic extension arrived worked like a dream, did,nt know whether to laugh or cry so had a very big alcoholic drink.
Many thanks for all of the tips etc
FYI it was part No 60 on the schematic