I would guess that there is no problem but really not enough information.
Line and Neutral are both considered as Live however with TN supplies one is allowed to switch only the Line to isolate. So as to if orignal isolator is concerned it will depend on supply type. Personally I would always want a double pole isolator. The problem with single pole is if the element fails then likely you will not be able to reset the RCD so I would say leave the double pole isolator in place.
The N on new switch will have both the N-in and N-out wires connected to it. The N is purely to power the device the Neutral is not isolated.
I would consider where you need to ask the question then you don't have the skill required and I would say your better off to get some one to fit the device for you as so easy to miss some safety feature.
Maybe your right because that all sounds very complicated!
Im very practical and leading from my tank are just two cables, one to my econ 7 on/off switch and one to my booster on/offswitch. All I wanted to do is put a timer in place of the booster on/off switch as i keep forgetting to turn it off! I thought all I would need to do is remove the booster on/off switch and run the wires into the timer instead. Or run wires from the booster on/off switch to the timer all be it I didnt think this would need to be done as the timer has its own off/on button
If fitted correctly the Economy 7 immersion should be set lower in the tank to the boost one and also the thermostat should be set higher than the boost one so under normal use even if you forget it should not activate.
Fitting a time switch as well as isolator would be no problem. Instead could present a problem if RCD protected.
The old mechanical timer was best for what you want. It had on and off pins and all you did was use only the off pins so manual on auto off. Something like the Screwfix Product Code: 91462 which you can set to switch off every couple of hours so you will not be able to leave on.
Today the Electronic Immersion Heater Boost Timer is produced specially to do what you want. TGBT4 up to 2 hours and TGBT5 up to 4 hours boost. I looked on the tlc-direct site and these are double pole i.e. they switch the neutral and are same size as standard socket or switch so likely direct replacement. At £16:30 plus vat not too expensive. More expensive that mechanical type but so simple to use. Push button and then forget it switches off after set time.
Yes my economy 7 is the lower of the two. However, I found that even with a jacket on the cylinder a night of heating the water econ 7 would only give one bath in the morning and not enough for one in the evening. I have electric heating. And live in a smallish two bed flat.
I have now fitted an electric shower so really I just want the upper boost on/off timer to be set for 30mins in the morning and 30mins in the late afternoon. (I always forget to turn it off and its on for five or six hours at a time on occasions!)
I guess the electronic timer I bought would do exactly as you say and as it only had three places for wires to go and would be a direct replacement for my booster on/off switch
I wonder if you would mind giving me the model name and number of the electronic timer you know of please and I will order it. It will mean I can take the wires from my top booster on/off switch and wire it directly into the new timer switch?