Thanks everyone I have found he problem, although I don't understand why it was. One of the parts had small glitch at the very start of the automation for the tube compressor input. I removed this and so far it seems ok. Although I don't see why this should have caused problems with other tracks in the mix.
[As an aside I recorded a pedal steel being played though a two channel volume pedal. Once channel had a constant level sin signal going into it, the other the instrument signal going to the monitor amp. I recorded the direct output of the instrument before the volume pedal, and the sin output that had been through the pedal. This meant I had the direct instrument sound and a separate pedal envelope, which I joined in the mix. I extracted the envelop using the analyst effect. The glitch was at the very start of the extracted envelop!]
Thanks for the various prompt responses.