We went to Masters yesterday (as a change of scene from Bunnings).
Our main mission was to purchase a replacement hose for the caravan. We actually bought about three metres, which is enough for two lengths. One to link the water tank to the sink tap - and the other to connect onto the tap and direct water outside the van if desired.
Nick re-fitted the pump tap inside the van today, attached to the new hose.
After discussion we decided to clean the water tank before it was re-fitted under the caravan.
I'd done some online research and explained a carb soda cleaning method to Nick. As it was an ingredient on hand, he decided to use that. The water tank has now been flushed and with luck will be re-fitted under the van tomorrow.