shower
I don't know how, but my partner installed our electric shower himself and I think my father did the same in his house (both of them too mean to get the job done properly!) In the case of our house, the plumbing and wiring was there for the shower so all my partner had to do was the actual connection to both of these.
Is the shower you already have run off the bath or just a shower run off the immersion? My parents have another shower in their house that is a power shower and they have to turn on the hot water on the (oil) stanley range a half an hour beforehand, unless the heat is on anyway. They always had to have their own awkward way of doing things!
In my house I only have an electric shower in the ensuite and the bath in the main bathroom but I want to sort out some kind of a shower for the bathroom, because visitors have to go into our bedroom to have a shower and also my lovely partner blames me and Triton for the high ESB bills, but the tap in the bath is too slow. It does seem a waste having an electric shower when there's hot water in the tank as it is and I heard something about having 2 electric showers in one house being dangerous???