It is quite common for the fused connection unit to develop arcing contacts, so this is where I would start.
13A devices get quite warm at full load, which in the case of an immersion heater is for long periods. I only changed a Contactum arcing FCU last week, on an immersion heater circuit.
If you currently have an FCU then this is what you should replace it with in preference to a 20A DP switch.
It is likely there is not a dedicated circuit for the water heater which means the 13A fuse is necessary for overcurrent protection.
Finally, 1.5mm heat resistant (preferably butyl) flex has an adequate current rating for a 3Kw immersion.
13A devices get quite warm at full load, which in the case of an immersion heater is for long periods. I only changed a Contactum arcing FCU last week, on an immersion heater circuit.
If you currently have an FCU then this is what you should replace it with in preference to a 20A DP switch.
It is likely there is not a dedicated circuit for the water heater which means the 13A fuse is necessary for overcurrent protection.
Finally, 1.5mm heat resistant (preferably butyl) flex has an adequate current rating for a 3Kw immersion.
