WDB CLRED TO LAND BEHIND WDB DETERMINES NOT COMFORTABLE WITH SEPARATION. MAKES GAR.

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ON APCH TO RWY 8R AT GATWICK INTL ARPT; UK. ALT LESS THAN 800 FT MSL. TWR HAD CLRED A 747 FOR DEP PRIOR TO OUR LNDG. WITH LESS THAN 3 MI TO TOUCHDOWN THE 747 HAD NOT COMMENCED A TKOF ROLL. THE CAPT AND I DECIDED THAT THIS WAS NOT ENOUGH SEPARATION FROM THE DEPARTING HVY 747. WE ADVISED THE TWR WE WERE EXECUTING A GAR. OUR INITIAL INSTRUCTIONS WERE TO CLB RWY HDG TO 2000 FT (PUB). WE WERE THEN INSTRUCTED THIS WOULD HAVE BEEN A 270 DEG TURN. I ASKED THE TWR TO CONFIRM HE WANTED A 'L TURN' TO 180 DEGS. HE RESPONDED AFFIRMATIVE 'L' TO 180 DEGS. AS WE WERE CONCERNED WITH OUR FLT PATH AND THE DEPARTING 747 WE ELECTED TO EXECUTE THE TURN WITHOUT FURTHER QUESTIONS. IT WAS CLR; VFR AND NOTHING SHOWING ON THE TCASII. WE WERE HANDED OVER TO APCH CTL WHO ASKED WHAT HDG WE WERE GOING TO. WE WERE INSTRUCTED TO STOP OUR TURN AT 240 DEGS. WE WERE SUBSEQUENTLY VECTORED BACK FOR AN UNEVENTFUL LNDG. UPON TAXI IN; TWR REQUESTED WE CALL THE WATCH SUPVR. THE CAPT DISCUSSED THE GAR WITH THE SUPVR WHO INFORMED HIM THAT THEY ROUTINELY CLRED PLANES TO LAND WITHIN 3 MI OF A DEPARTING HVY ACFT. THE CAPT AND I BOTH FELT THAT A 747 CREATED TOO MUCH POTENTIAL TURB TO LAND THAT CLOSE BEHIND; ESPECIALLY IN A LIGHT WIND SIT. (LESS THAN 5 KTS). THE SUPVR THEN TOLD US HE WAS GOING TO FILE A FLT VIOLATION BECAUSE WE HAD TURNED THE WRONG WAY ON THE GAR. HE SAID WE HAD BEEN INSTRUCTED TO TURN 'R' TO 180 DEGS. SUBSEQUENT INVESTIGATION AND PLAYBACK OF THE TWR TAPES CONFIRMED WE HAD INDEED BEEN INSTRUCTED TURN 'L' AS WE HAD INDICATED. THERE WAS NEVER ANY TFC CONFLICT THAT WE WERE AWARE OF. THIS WHOLE EPISODE COULD BE AVOIDED WITH SPECIFIC GUIDANCE AS TO LNDG SEPARATION FOR A DEPARTING HVY JET AS WITH THAT OF AN ARRIVING HVY JET.