It was taken from the manual for the 600 series system boilers!Your diagram is for a combi, G_Hailsham.
The point I was trying to make is that the diagram showed the supply to the clock coming from the spur/L terminal, not terminal 3. The reason for this is obvious - terminal 3 is only live when the boiler is switched on. This does not matter as far as a room stat is concerned, but would be unhelpful if a clock was connected to terminal 3.
There is a warning in the manual: Mains connection terminals L and N remain live even when the boiler on/off control is switched off.
The OP has a 615.