Thanks very much - we'll investigate further and keep you posted.
The tank is metal, so the heat in summer may be a problem (but it is under the shade of a big tree). We can see problems in cutting out doors/windows etc. (angle grinder?) but the attraction is that it MUST be fox-proof!
PS did anyone else see the half morris traveller in this months Gardener's world? That's what lit the spark of an idea......
NN