However, I had a slight brainwave.  Instead of a 5t tank losing 10W (10% loss of stored heat over 6 months), why not use a 10t tank losing ~100W (~50% loss of stored heat over 6 months)?  The extra energy to put in the tank is readily available in the summer, and I need only ~10% of the insulation, ie something like 50cm instead of 5m+.

The downside is that we'd definitely need to dig down to accommodate a large enough tank as we just don't have lots of ground space.  But we might not have to dig much.

Rgds