AerBabe,
The FG code is a qualifier for the type of obsuration of the visibility, which always immediately precedes it and may be smoke, sand, dust, etc. It is really coincidence that in this case, the cloud was what was causing the reduction in visibility.
A cloud report of OVC000 indicates that the cloud ceiling (what you & I might call the cloudbase!) was less than the minimum reportable value of 001, or 100'. Perhaps it was just possible to discern the base of the cloud. More usually in visibilities of less than 600 metres, the report would be VV////, or 'sky obscured' because it's not possible to make out the bottom of the cloud. Perhaps the horizontal viz was around 900 metres, borderline for being mist, which strictly starts at values in excess of 1,000 metres.
Have a look in the UK-AIP GEN 3.5 pages 30-39. It's free to view or download at
www.ais.org.uk.
Hope the wx improves for you soon! Saturday pm might be better, or Sunday am, providing it's not too windy.
Must be great flying regularly at Duxford, enjoy!
Cheers,
The Odd One