(sorry :-})

It isn't up and running yet (still isolated from the water supply),
but the safety override switch was pressed in while the moving bits
were checked for operation.

Should it pop back out again immediately once the thermo unit is
dropped back to a lower temp ?

The reason for asking is it's sticking "in", and we're not sure if
it's a heat related function that'll sort itself out when it's plumbed
up and reached temp...