"Geoff Berrow" wrote in message
...
I've just bought a house which was built in 1965. It has a modern
consumer unit but having just removed a light switch I notice the wires
leading to it are black and red.

At what point does a rewire become necessary?

When it's a) illegal (which yours might be) or b) before it's dangerous.

Don't risk it, install modern wiring, regulation compliant.

Maty