OK. So I've bought another control unit to try due to the absence of any further advice. It has the same exact same part number so I presume it does not need reprogramming (the unit states it requires re-programming, but I guess that is for brand new units). And this time i will disconnect the battery first !

Should the the light go out once replaced even though the error will remian in the ecu until it's cleareed ?

Would a different error show up if the problem is with the clockspring ? most modern cars with OBDII would, but as the Fiesta is one of the very first diagnostics on board using the older tech - needing a special diag tool and software. The commecially available tools on Eb*y would not work - only on the later vehicles 2002 onwards.