This has all re-kindled my interest in reverting to a thermal store
system,
though, but I still haven't established how the dhw loop works when the
hot
is really just heated mains water. I cannot find any reference to dhw
loops
on the Albion site.

It makes no difference what sort of system you have when fitting a
circulation loop. When all the taps are off, the pressure at the cylinder
and the pressure at the tap are the same (ignoring height effects).
Therefore, a simple low power, low head pump can circulate the water back
whether it is gravity fed at 0.2 bar or mains fed at 3 bar. Just a tiny 0.1
bar pressure differential would be enough to recirculate in each case.

The main problem I could see with a heat bank is that running the exchanger
pump would cause destratification. However, I wouldn't be surprised if they
had designed the system to deal with this issue. It may even be that
parasitic effects are enough to keep the recirculation loop hot, without
needing to run the plate exchanger pump, but I don't know how it is
designed.

Christian.