Horstmann e7 quartz

Have one of these immersions.

The night switch is tripping when it comes on. I assume the element is gone from a quick google search (?)

I couldn’t see though how much it would be to replace the element (ie the main element, not the booster – the booster still works). Any idea?

The element has split in the immersion. You will need to replace the whole immersion heater. You cannot buy an element on its own as it is part of the overall immersion heater itself. Cost will depend on the type and length of element. You local plumbing suppliers will give you a cost.