Update!
The problem turned out to be a worn compressor and a slightly flat battery. The latter was easily resolved but the compressor turned into a saga.
I had a new piston ring in my spares stash which I seem to remember buying because it came with free desiccant for the drier. I fitted the new ring and the message was still there. On the bench the compressor struggled to reach 5 bar never mind 14. A new, proper ring saved the day , restored compression and broke the daily routine of removing the compressor. We never have time to do a job properly yet find time to do it twice.....or three times.
The moral of the story is 'Only buy the original Bagpiping Andy kit or risk becoming very efficient at removal and replacement of the compressor.