You will have to trace the pipes from the boiler! At some point they will divide, with one branch going to the rads and the other to the cylinder. The valve will be on one branch or the other.
This needs putting right. Can you do electrics?
In your previous topic, you asked about wiring the ST6400. Did you use the same cable as the old programmer, if so how was it originally wired? Or did you use a new cable, if so how many cores does it have - excluding the earth.
Can you check the wiring at the boiler end. You should have three wires, brown, blue and yellow/green, going to the br, bl and gy terminals. These should come from a permanent live source. Usually terminals 1, 2 and 3 of the wiring centre. There should also be two wires, brown and blue from terminals 1 and 2 to the L and N terminals of the new programmer.
Finally, you need a wire from terminal 8 to the D terminal in your boiler.
