Horstmann Economy7 Quartz Boost not working?

I have a Horstmann Quartz Economy7 timer which controls a dual element immersion heater.

When the main timer switch is in the "on" position, the "off peak" neon light comes on immediately (even though the clock has not yet reached the off peak time). The bottom immersion heater in the tank only activates overnight at the correct times.

If I try to use the Boost function, the neon light only briefly flickers but does not stay on and the top immersion heater doesn't come on. I've had the tank drained and the top immersion heater replaced (even though the old one was found to be ok and was not furred up) and I've tried swapping the top and bottom thermostats around but the issue persists.

The electrics look ok (visual check only, no lose connections) and the main fuse breaker switch has not dropped.

I'm trying to understand if it could be a faulty timer. Is it normal for the "off peak" neon light to stay lit up at all times?

M
August 2020