Jeff - I have done some low-key research (ie messed about on Google) and so far come up with various possible pipework configurations and some suitable products. I am looking at "Vetus" tanks which come in a suitable size. They are not eye-wateringly expensive, but seem to have the right specs - see Vetus Flexible Waste Holding Tanks at AhoyCaptain.com - Always your best source.
The main problem with flexible tanks seems to be the danger of wear caused by rubbing against surfaces, projections etc and my next task is to look a a suitable material to put in the under locker spaces which will protect against this.
Leith (rhymes with teeth) is the port of the City of Edinburgh in Scotland. A Leither is someone who comes from that area.
I must go down to the sea again, to the lonely sea and the sky - I left my shoes and socks there, I wonder if they're dry?