I'm trying to put up some new wall lights and it would appear that
the three wires are not correctly identified - can anyone tell me
a simple way to find which of the two low potential wires is earth

Throw the double pole isolator on your consumer unit and unplug every
appliance in the house, particularly those with metal cases, such as white
goods and computers. Turn off any immersion heaters at the isolators. Then
resistance check the cables to be tested to nearby radiator pipework. The
neutral should show a high resistance, whilst the earth should show a low
one. If you don't isolate the neutral by throwing the isolators and removing
the plugs, you will also get a low resistance in the neutral, so won't be
able to tell the difference. If you have plastic pipes, or both tests are
high resistance, test to the earth terminal of a 3 pin socket.

Out of interest what would the consequences be if I connected these
incorrectly (no RCD or earth leakage trips in the fusebox)

It would work, but it would be dangerous.

Christian.