The message
from Geoff Berrow contains these words:

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.

You could have bought a house built in 2005 and still made the same discovery.

At what point does a rewire become necessary?

These days not very often. If you had rubber covered wiring you should
be concerned but that should not have been in a 1965 new build.

--
Roger Chapman