http://www.flyertalk.com/forum/qatar...z-economy.html
MH's Golden Lounge is open from 3.00 to 21.30 according to the OneWorld website.
According to the MH website, the MH lounge opens 3 hours prior to departure.
Maybe a little search on other FT forums will give you the info. that u need.
As BA Gold, I believe you have access to the SkyTeam lounge at LHR T4 as well.