Hello Tom,
I had 2 Plastimo flexible tanks fitted under the quarter berth - a rectangular 100 lt and triangular 120 lt?. I found them too large so now have just the triangular one under the berth next to the engine.
A friend with a P27 had a tec-tank (solid sides)made to fit the same place so they must have the pattern?
Biggest problem was tainted water - which came from the water pipes supporting algae growth where they were in even low light levels. Found some flexible pipe made for potable water devices (coffee machines etc) which has white outer and a black inner so is not supposed to allow the algae growth. Won't know if it works until next season though.
hope this helps
cheers, ChrisN