Thanks guys. When I first saw the bilge pump reference (on my idiotPhone) I got nervous - I thought SW was when the flats dried out, not when they were flooded but then I clocked the reference to shifting water from tanks; I like he annotated pictures too though having three in a boat is either extreme conditions or redundancy or a leaky boat. As long as the leak is not at the end of the boat I am sitting at I am okay.
I am still exploring (cost effective) ways to get a rad in a tank; I had a minor light bulb moment when I realsied a specialist made me two fuel tanks in alloy for the Lotus Europa for less than the rad people were going to charge for the water tank so I have sent an email - I could do it in fibreglass but there is a cost vs. time equation here too.