thanks both for replies. Quick update:
Yesterday (when fault present) I had checked all fuses and disconnected battery (briefly) and shorted battery leads together - problem remained. I had planned to try the "try starting, leave ignition on when it stalls and disconnect battery for many hours, and should be cured when battery reconnected" method.
However, when I went to the car this morning, the fault was gone! Dash was lit up as normal, and it started instantly and ran fine. Nice, but would have preferred to have found a fault.... I therefore removed trim and cluster, checked fit of plugs (both ok) and did a brief inspection of cluster on kitchen table (removed casing, inspected soldered joints , but didn't remove needles and faces). didn't find anything conclusive. I then did some testing of connecting each of the two connectors in turn and ignition on to see if i could replicate the fault. disconnecting the smaller block (power/earth feeds?) gave the same result (dead display). It did however throw an airbag light fault/light up, but that was cleared with a cheapo ebay VW OBD code reader which i was pleasantly surprised about having read posts about needing VCDS or dealer.
So with it all back together for now, I began thinking about cause. It was -11 here yesterday morning, but car started fine having been sat for a week. There was moisture on inside of drivers window which this fabia hasn't had before (rare i know, i have two other very damp fabias...). It was driven for about 10mins with heater on maximum before gauges froze and display went out, but carried on driving ok. Once stopped and switched off, it then would not restart (well started and immobiliser cut out after 2secs). Then 3hr wait for AA to not be able to diagnose and truck it home. This morning was a mild -2, and dash was lit up fine ok and so it started and ran fine.
So first i thought cold and damp related, but it started fine first thing both days. Then i thought heat of the heater on max, so i sat in the car for 30 mins with heater on max today and tried wiggling/pressing connector with console loosened, but i could not recreate the fault (shame).
That's as far as i got today. It works, and Airbag light out, but I'd prefer to know what the issue was and fix it.
Is it possible to provide a 12v and earth to the cluster on the bench, and then try some heat (hairdryer) to seek out any dry-solder joints? Has anyone tried that?
thanks.