I have a Santon Premier plus unvented water heater with just a single heater and coil from the central heating. Is it possible to install a dual heater/element for Bath and Sink heating?
It would appear that the particular water heater that you refer to has a low level side entry immersion heater, designed to heat entire contents.
The Bath/Sink Dual immersion heater can only be inserted from the top
of a cylinder. Only possible if a second plugged immersion flange is on top of the cylinder. Unlikely