Hmm.
well actually it was none of those things - and while I know exactly what happened (just not why), so far there isn't a sensible explanation.
When I deleted the existing Forum folder and created a new one - the new one showed up.
UNTIL I put files into it and changed the chmod to the same as all the other folders and files (755 & 644)
At which point the Forum folder vanished from html again.
Much banging of heads on walls insued.
However, being an afficianado of murphys law I decided to ignore what 'should' be and just create the Fourm folder without any changes to the chmod. And that worked.
I then uploaded the template and css files - again without messing with the chmods. And they remained visible to html.
So currently I've got the folders and files with default chmod and they are visible.
so folders are 775 and files are 664.
If I change them they turn invisible (still there bill, just invisible).
ALL other files and folders including the Templates/Admin folder are standard chmod 755 & 644
And so far it's working.
Before I moved the Templates/Forum folder had 755 & 644 permissions. Go figure.
So thanks for all your help guys - even if an idea isn't right, it got my brain thinking in the right direction.
You just need to remember with computers - what 'should work' be isn't neccessarily what does work
So just got to get my navigation menu back (and I saw a thread on just that subject on here

) and I think we'll be back to normal.
Cheers guys - see you when the next problem crops up