I have a Megaflo DD170, which was working fine until recently.  I'm not getting hot water on timer, but the boost button is working.  ie am getting hot water from top immersion but not from bottom one.

The cut out on the thermostat has not tripped, but I've tried switching the two thermostats anyway, but no luck - the bottom immersion is still not heating the water.

Any ideas?  Problem with megaflo unit or with timer? (timer clock is going round & light comes on as if its working ok) ie. should I call a plumber or an electrician?

Any help appreciated.  Thanks in advance.

An electrician to check the immersion heater

I agree with Scruff - At 4 years it could well be the element that's gone kapput, it could also be the timer has developed a fault. try to heat as little water as possible until you know one way or other, if the element needs changing it's a drain down.