WHATEVER are the night owls going to do now that Roger Thomas, Ted Marr and the rest of the nocturnal talking heads on lately-defunct Metro News have been taken off the air? And it's not just Hongkong's insomniacs who are hurting. There are quite a few back in London, too, who tuned in to Roger rabbitting on about Hongkong via a live hook-up with LBC - an all-night station in the British capital. Ironically, just days before the plug was pulled, a lady in Tunbridge Wells wrote to the seasoned broadcaster, datelining her letter ''3 am LBC'' and addressing the envelope ''Roger Thomas, Radio and Television, Hongkong''. Needless to say, it got to him. So there's fame for you! ''You mentioned all the things I wanted to know about,'' she wrote. ''It was super, and you sounded as if you were just around the corner.'' (Knowing the ubiquitous Roger, he probably was.) Later in her letter, she asked if he might know anyone fleeing the Basic Law and its attendant evils who desired a ''little place in the country'' in picturesque Kent. Enclosed were complete details of the property together with photographs. The letter couldn't have been better timed for the erstwhile Metro man. With one sideline now scuppered, perhaps an alternative in real estate beckons. On the other hand, with programmes like Roger's ConversAsians eliciting such positive feedback, could it be possible that another radio station just might take up the idea? Worth a thought.