Having installed a combi boiler recently we no longer have the backup of the
immersion heater in the cylinder. So we are thinking about installing an
electric shower so that at least that will continue to work should the
boiler not be working for any reason.

However since we are having to modify the bathroom to take the new shower I
would like to make regular use of it, not just in emergencies, it would
probably be best in that case for the shower to be able to run off the hot
water supplied by the boiler.. but should hot water not be available it
should be able to generate its own.

Is there such as thing as an electric shower which can use a hot water input
when it's available?

Cheers