Russ wrote:
"direct" as in it's got an immersion heater only

"indirect" being the tanks with the additional connections on the side
and the coil inside, into which you'd feed water from the boiler.

All we've got is immersion heated water.

then having a heat exchanger inside it will have no effect. You could
use an insulated removed used one, which would not comply if heated
indirectly.

NT