Thank you Andy for a comprehesive reply.
I feel I could be looking for a needle in a haystack without knowing what a needle looks like so I've sent the unit off to a independent xerox engineer to invetigate.
Today the Fiery just spins up the fans for a couple of seconds and then dies.
The most common failure of the integrated fiery is the processor (CPU) fan. It will slowly begin to fail (not want to run at full speed, or will intermittently stop). This results in frequent fiery reboots or disconnects. And eventually leads to a damaged processor. Obviously dust accumulation can also be a problem for cooling to be effective.
With power disconnected from the fiery, open the fiery case. You should be able to spin the cpu fan by hand - it should continue to spin freely and gradually spin to a stop. If it seems stiff, or does not want to spin freely by hand, then it needs to be replaced.
The processor may have over heated due to intermittent cooling. This may be why it will only remained on for a short time before turning back off. In that case, the cpu will also require replacement.
The hard drive will sometimes fail as well, but the fiery will usually remain on much longer than a few seconds if the hard drive or sw has failed to load.
The mother board can also fail, but much less common.
Hope this helps
Recently, on a couple of occasions the Fiery controller stopped booting up when switching on the printer. I pressed the green led button near the power supply and all worked fine.
Today the Fiery just spins up the fans for a couple of seconds and then dies. There's clearly power to the unit but I'm not sure why it's decied to stop working. It's been running fine for 3 years or so.