Thanks Peter, sounds like a good idea, I will try it. Also thanks Alan for the Littleport tip for fendering. After a lengthy wait I finally have a sample which although a smidgin too wide, is, I'm sure, as close as I'm going to get.
Reagrding my question about a gauge for the water tank, i have discovered that Tektanks do a system where you can fix sender units to the outside of the tank and check the level. I haven't fitted it yet but I will report back when I do.