Ms Derrick   THE PE DEPARTMENT   Ms Martin    BBC LONDON 94.9FM   BBC LONDON is to Mel and Sue what Rio was to Ronnie Biggs; a home from home, an escape hatch, the proverbial bolt-hole. It is also a paid gig.       Are you in on Saturday morning? Perhaps you're hung-over, you're laid up sick, or you're serving time at Her Majesty's pleasure. Whatever the reason for your being inside, alone on the most exciting day of the week, why not tune into 94.9 FM or LISTEN ON-LINE and be part of the 'fun'.       From 12 - 2pm every week, Mel and Sue will be hand-picking the finest guests from the orchard of media, dragging them out of their beds at an ungodly hour and then asking them silly questions which belittle their status.       There's a regular competition - Guess The Sit-Com, 'Hilarity', live phone-ins, the legendary soap opera PE DEPARTMENT and random sketches featuring unfeasible and ludicrous accents.       All of this, plus London's coolest tunes which, thankfully, someone younger and hipper has chosen for them to play.