Hi guys
Thanks for the comments and pictures, all very helpful especially the colours.
Derek,
The ferry was designed primarily for trains or vehicles or carriages, there are no below deck cargo storage other than the ship's fuel bunkers.
As far as I can see from the admiralty plans most of the space below is engines, boilers, coal bunkers and split accommodation for officers and ratings, nothing mentioned for ballast or a system to keep the deck at a certain height. Many videos show a very wet deck at the aft section, and Sandy (Yarpie) confirmed this with his pictures too.