Thanks for all swift responses.
Tigger is a 3 cabin version. So the heads is at the "front", i.e. between the galley and the stern starboard cabin.
Therefore a locker fitting would not really help/be easy (long runs). I am thinking I would have to encroach into the head's space.
The shape is rather awkward. Plus I rather like the clean, uncluttered feel… a tank there would definitely "encroach"