On Mon, 01 Apr 2013 16:10:30 +0100, Roger Mills wrote:

But, as others have said, if I power down, disconnect all 3, power up
again and check which 2 wires are live, I will know which is the spur -
regardless of what it does.

Well you can but I quite like the power down, disconnect all three and
then use a meter to see which lives are still connected to each other. No
second trip back to the CU, no live working. The two still connected to
each other are the ring, the isolated one the spur. The fun starts when
they are still all connected to each other via dead shorts...

--
Cheers
Dave.