Hi,
This could be a different issue but there's a lot of similar problems on MK4 Ibizas caused by dodgy soldering ( from the factory ) on the instrument cluster. Re-flowing the joints on the main 32 pin connector fixes the problem in the majority of cases. It only seems to effect the Ibiza Sport with the 6 dial cluster but it could be a similar problem on the Fabia ? I got quoted about the same from Seat to fix mine, £450 for the cluster and £200 to fit and re code it. Luckily I found a thread about it on the Seat Cupra forum that explained how to remove the cluster and re solder it the pins. Others have had problems with gauges going off or doing strange things as well as the immobiliser and it all turned out to be the soldering on the connector.
http://www.seatcupra.net/forums/showthread.php?t=199145
http://www.seatcupra.net/forums/showthread.php?t=358596
( hope I'm not breaking any rules posting links to other forums, they're purely just to try and help )
I was quoted around £85 from a local tuning company to do it but gave it a go myself and 2 years later all is well ( touch wood ! ).
As I said it could be a different issue altogether on the Fabia but you never know.
Al.