as I've paused the arma 3 beta 'dev' update, the game no longer shows in my installed games listing therefore have now reinstalled steam from the steam website, reinstalled arma 3 beta, opted for 'dev' then updated as usual.
strange thing is now my bohemia/arma 3 account has been blocked (possible hacking?), so have now reset my password, and is now all working fine again.
my theory, arma 3 account hacked, dev updates thus blocked, steam affected somehow by the hack. then again it may just be a corrupt install of arma 3 beta.
over and out!