I have been using an NCE PowerCab for probably 15 years but have never really understod how it works.  Yesterday I was running 2 new sound locos - Dapol Flying Scotsman and Dapol 92220 and both worked fine then just stopped.  I tried 2 or 3 other locos but they won't run.  My handset is live and doing all the things it should do.  I have got TrainTech buffer lights fitted and they are alight.  Has anyone any suggestions of what I can check or is there an easy way of wiring it up to a test track?

I suggest you disconnect the track output from the PCP and connect up a piece of test track. Then use a non-sound loco on this test track and see if you can read CV values. Use Programming Track and Direct as the selections. I have a very old Powercab which is now a standby, but have never experienced this behaviour when I was using it for layout control. I can envisage a situation where the track voltage is present, hence buffer lights on, but no DCC signal is being sent.
Can you try your locos on a friend's or club programming track?
HTH