Single pole or dual pole relay for switching a 3 kW water heater?
All new immersion heater thermostats in the UK have a secondary over-heat cutout with a manual reset as a precaution against the primary stat failing.

If the existing stat does not have an overheat cutout it would be wise to replace it with one that does. They're readily available from electrical/plumbing places and in most cases slide into a sleeve in the heater and do not require draining down. 

A single-pole relay will be sufficient.