Mike - I wrote a long post on this which seems to have been lost when the forum server crashed a while back. I found the problem was in the IDC connector that connects to the immobiliser coil. IDC = insulation displacement connector - this is when the crimp contact connects to the wire by cutting through the wire insulation. I.E. it's a mechanical connection, not soldered. The wire to this connector was very taught which had caused a bad connection at the crimp. As it was constantly being pulled. I soldered each crimp to each wire (2 in total) to make a good connection. Also pulled some of the immobiliser wire back through towards the coil and cable tied this to the barrel to try and stop it becoming tight again. Think this happens over years of steering wheel adjustment.
The coil is around the lock barrel and is only accessable by removing the wheel, which also involves removing the airbag (be careful) The airbag has two bolts through the back of the steering wheel and a single connector that pulls off. The wheel needs to come off to get the steering column surround off.
Good luck.