The diode across the relay could be a 1N4001 type or equivalent.
You say that the Override switch is in the Normal position on the drawing you posted.
This would mean that the relay/pump is always On and the sensing circuit would never control the relay.??
I would say that if the Override switch is in the Normal position the circuit would be in sensing control operation, not manual Override.
Look at this modified drawing.
Also the 10K base resistor to the BC547 will only give approximately 1mA, so with a saturation gain of the BC547 of say 20, thats only 20mA through the relay coil,, what is the resistance of the relay coil.?