In other words, I though that the termal store system wasn't dissimilar to
the way that a conventional HW cylinder is pressurised by the CW tank in
the loft. So I hadn't actually foreseen a problem, as long as the rads
were below the header tank level.

Yes, and you would need to use a similar system in your setup.

However, in certain circumstances (such as mine), you can take advantage of
the fact that the water goes nowhere to remove the expense, space and
inconvenience of having the header tank. In your case, I don't think you can
and you need a conventional vented tank arrangement.

Is this an area which thermal stores vary a lot in, and does "sealed
pressurised system" mean "fed from a header tank in the loft"?

No, it means a sealed circuit with no vent, filled from a hose from the
mains, with a pressure gauge, pressure relief valve and expansion pressure
vessel. There is no need for there to be a boiler in circuit to have one.
You can run a sealed pressurised circuit from the heat bank, with an
additional heat exchanger, but it adds to complexity.

Christian.