D1 and D1 are not needed. In your diagram D1 is shorted out. Back to your other question about the black capacitor, I don't know. The schematics were drawn after the fact many years ago. I've had to move files three times to new computers and all of the circuit board layouts have disappeared. If the external wall wart in a 12VDC regulated switch mode power supply you don't need C1. And if you are using a bigger relay than my reed relay, you need a diode across the relay coil. Cathode to positive (arrow pointing up).
Nigel,
I'll try to take the time to shrink what I post. I really liked .gifs for schematics, but the current photo editing programs on this computed don't give .gif as a SAVE AS option. I do find .png in PhotoShop.