I have a 16 amp 230V supply and wish to auto disconnect heating when using a kettle using a relay,
You could use a current transformer with its primary in series with the kettle and its secondary connected across the AC coil of the relay/contactor you're going to use to kill the heater while the kettle's plugged in. 

The relay contacts would be normally closed and they'd open when you plugged in the kettle. 

Caveat: There'll be an instant when the kettle and the heater would both be on until the relay contacts opened.

More important, if you don't know what you're doing and/or you're not familiar with working with the mains, you could easily kill yourself, so my advice would be to hire a licensed electrician to do the work.

There are also devices called "current switches" or "current relays" which will do exactly what you want, so you may want to take that course instead. Search on Google for lots of hits.