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

Well, you could run the outlet of a flush mounted thermostatic mixer through
one, but why would you?

Electric showers are ugly and seriously constrain the water flow.

If you really need a backup, consider using an inline electric water heater
on the hot feed to a standard thermostatic mixer shower setup. This way you
get nice looks, good combi performance and can just switch on the electric
(and turn down the flow) when required. Careful choice of location should
enable all the taps in the bathroom to benefit. You could even drive the
kitchen off it, although I wouldn't advise it unless the bathroom and
kitchen are close together, or you'll be waiting an age for the water to
come through. Select a good thermostatic mixer that claims compatibility
with combis, as these should be able to cope with the effect of varying
temperature on the hot side.

http://www.hygienesuppliesdirect.com...d=46&storeid=0

You could probably get away with the cheaper 7kW, as it is only an emergency
backup. You won't mind the slighter drizzle if it is saving you from having
to use the showers down the local swimming pool! On the other hand, if
you're running new cable and 30 quid means nothing to you, go for the full
9.5kW.

Have an electric shower head available. These will work better on the low
flow electric situation than a standard power shower head that you should
use when on combi power.

Christian.