First of all read the installation instructions IN DETAIL. Specifically look for information about using it on 3 wire or system with a equipment grounding conductor.

Also post the brand and model number.

” Our supply has black, white and earth.”

Exactly what kind of wire or cabling or conduit system was used. And how have you identified one wire as “earth”.

We get few messages from the UK and since you used the term earth are you in the US or not.

And what kind of breaker to those wire connect to. Single pole, 2 pole, and does it also control the cooktop.