The new shower unit requires connection via a 3A switched (double pole, 3mm gap) fused connection, along with a 30mA RCD somewhere in the circuit. At present there isn't an RCD fitted to any of the house circuits.
We currently have an old 8.5kW electric shower that will be removed. It's on a 40A plug-in MCB in old Wylex consumer unit which comes up the wall cavity into the loft. I would like to re-purpose this radial circuit for the immersion heater (single, 3kW) which was replaced previously but never wired up. The previous one plugged into a 13A socket in the airing cupboard, spurred off somewhere (I presume the upstairs ring). The plug used to get hot and I didn't like the idea of it being off the ring. Hence when the shower radial circuit is freed up, it would make an ideal feed for the immersion heater.
Would it be okay to run the shower off the same radial circuit via a separate 3A FCU? That circuit is currently running 8.5KW of shower.
As the cable is in the loft and easily accessible, I intend to fit a RCD (1G box mounted) for the shower outside the bathroom in the landing area close to the ceiling. This would allow us turn off/reset the shower (in case of any problems) by 'testing' the RCD - without having to get into the loft. The switched FCU for the shower would then be close to the shower unit in the loft, making it clear for person working on it to see if it's turned off.
I would do something similar for the immersion heater, with it coming off a 13A FCU mounted on the landing and a boost timer switch (e.g. http://www.screwfix.com/p/horstmann-e15-boost-timeswitch/67063 ) to show when it's actually on.
Should the immersion header be on an RCD too?
I'm trying to avoid having to touch the consumer unit, nor pull any new cables. Does what I've suggested sound a sensible DIY plan?