Wow, thats an old boiler, your lucky its still going. Yes, a wall stat can be connected to this. It should be connected in series with the feed from the programer.
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If your taking the new cable to the boiler then turn off (isolate the electrics). You need to run a 3c & earth cable. Remove the core from the S/L terminal and join this with the black of the new cable into a joint connector. Connect the new brown core into the S/L terminal. Connect the Grey of the new cable to the N terminal & the earth cable to the E terminal (other cables should already be in the E & N terminals). At the wall stat end the brown & black are your switch, the grey is the neutral and the bare earth is earth. The bare earth should be sleeved green& yellow, the grey sleeved blue & the black sleeved brown, all at both ends.
Not all room stats require a neutral.
