The wiring depends upon the type of heating that the thermostat is fitted to, as you seem to have a boiler, the installation method is to use 3 wires, as per the RTS8 instructions below.
If I have read your post correctly, it would appear that you have an existing LP241 and you don't have a thermostat, so wish to add an RTS8 to it. If so, you need to break into the existing wire going to LP241 terminal 4, and reroute it via the RTS8 and add a neutral connection to the RTS8. I have sketched this out for you below.
The wiring colours may not be applicable for your installation, but I have used them just show what is connected where. If the wire used as a neutral isn't blue, a bit of blue sleeve at each end can be used to identify it as such. Obviously I haven't shown the other LP241 wiring only the bit that needs changing. All other wires remain as they are.
Usually the three core and earth sheathed cable used for 2-way lighting circuits is used to wire a thermostat. The thermostat is operating at mains voltage so mains wiring is not an overkill, it is essential.
If you are unsure about anything, or I have misunderstood what you are trying to do post back.
EDIT: Thermostat location details below added.
The thermostat should be located in the most suitable place in your home for it to work properly. Here are some guidelines.
The room thermostat will switch off all of the radiators in the house when the set temperature is reached in the room in which it is located. Therefore, it should be in the last room to get warm. Understand this concept and you will be able to work out the best location for your home.
Because properties are different in use, orientation, construction size etc., it's not possible to say your thermostat should be located here or there. But consider the following.
Do not locate the thermostat in a room where it will be subject to other heat sources such as cooking appliances, used fireplaces, or sunlight. Any extra heat that warms up the room where the thermostat is located, will switch off the thermostat, and all of the radiators in the rest of the property.
It should be located in an open space to allow air to circulate freely around it so that it senses the true ambient temperature of the room in which it is located, not stuck in a corner, behind curtains or furniture, or subject to drafts.
Do not fit a TRV to the radiator in the room where the thermostat is located. if you do, it could turn off the radiator before the temperature set on the room thermostat is reached. In this event, the room thermostat will never turn off, and so there is no point in having it. [Some will fit a TRV, and either remove the sensor head or leave it set on maximum, but at some point no doubt someone would turn it down, so personally I would not fit one in the first place]
Fit TRV's to all other radiators to prevent the other rooms overheating
Ideally it should be in a place that is the last room to warm up, so that the radiators remain 'on' in all of the other rooms first to allow them to heat up before the main thermostat starts turning the whole system off.
