Also can someone explain to me the different kilowatt ratings, how does this work ?
More kW means more hot water per minute.
Bare minimum shower through a regular head would be about 8 l/minute. To do that instantly would mean nearly 20kW.
Electric showers work by using restrictive shower heads to give the illusion (thinly veiled) of a shower at about 4 l/min.
To get more water with less kW you need storage, but that just offsets the reduced energy input.
This is assuming a 35 degree temperature rise in water - which again is a bit crap.
Hence Muggles nicking my phrase "champagne ideas, beers money". - Muggles - it has been duly noted .
The only way I can think of getting your idea to work would be to change the shower head on your mixer shower for a spare one from an 8kW electric shower, but frankly i would rather smell than use it.
I can replace the mixer with an electric shower kit, as I say it's an option. It concerns me how easy this will be to do, how easy is an electric shower to retrofit? Obviously all I have to play with are 2 copper stumps protruding from the tiling in the shower after removing the mixer.
Looking at price I can find a 8kw+ shower for less than £100 and so if I find a reasonably powerful undersink heater that supplies the basin and sink I'm hoping this will work well. Thanks for the info.
Also just to say I made the mistake of buying and having fitted a sadia megaflow 125l cylinder, a fantastic heater but totally oversized for my requirements. It takes up the entire cupboard and storage space is very tight here. I plan on removing the cylinder for use elsewhere as I have a family member who can do with an upgrade. Shame really as I had to get in a plumber certified to fit the cylinder and sign off and now I'm taking it out to make space. I didn't know any better at the time and should have went with smaller more discrete heating