Timed selector switch

[Little Clanger]

I've been searching, unsuccessfully, for a timed switch that would change from one circuit to another.

I have two immersion heaters in a thermal store, fed by a single 2.5mm cable. I only want to power one immersion at a time - the lower one overnight and the upper one at specific times during the day. Something like a timed version of one of those bath/basin selector switches.

I've drawn a blank. Can anybody help?

Thanks

PS An ordinary Economy 7 timer wouldn't work because the 'boost' function isn't automatic