ARGGGGGG
I have spent the last three days trying to do this following old posts about overrides and if statements - and nothing seemed to work for me - so today I finally got the hump and gave up.
And then I saw this -and it worked a dream. Can't believe that no one seems to have highlighted this before? Thanks Kuroi -

BTW can this be used for sideboxes on EZpages? E.g. Can we use something like 'page&id=4&chapter=0' instead of a cat name like 'contact_us'? I couldn't get this to work - but thats no surprise based on my past record.