I did seem to find some cunning device on an American website that
consisted of a reservoir linked to a probe that sensed moisture levels
in the pot.

Did anybody see the Gardener's World Live show? Sarah had found a terracota
thing which was put in the pot with what looked like an upside down glass
ball which slotted into it. Apparantly kept plants watered for about 6
weeks. Does anybody know where to get these?