2 ACFT ACCEPT THE SAME CLRNC WITH SIMULTANEOUS REPLY IN FOREIGN OP.

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APCHING EGPF TALKING TO GLASGOW RADAR; EXPECTING RWY 23 (ALREADY BRIEFED AND CONFIGURED); RADAR VECTORS DSNDING TO 4000 FT; HEADING NE; IMC; WX AT GLA GOOD: 5 KM VISIBILITY; 2000 FT BROKEN; WIND 5 KTS; CAPT IS PF; FO IS PNF. INSTRUCTED TO TURN L TO 185 DEG HDG; RWY CHANGE TO RWY 5; VISIBILITY MIDFIELD 700 FT WITH LOW LEVEL FOG. DUE TO WX CHANGE; WE NOW BEGIN BRIEFING AND SETTING UP FOR A MONITORED APCH. BOTH OF US ARE HAVING TROUBLE UNDERSTANDING CTLR. WE KEEP ASKING HIM TO REPEAT. WE ARE VERY BUSY AT THIS POINT WITH BRIEFING; AND BOTH OF US HEAR CTLR ISSUE CLRNC TO DSND TO 3000 FT. FO READS BACK CLRNC; 3000 FT IS CONFIRMED SET IN ALT WINDOW BY BOTH PLTS; AND WE DSND TO 3000 FT. CTLR THEN SAID; 'CONFIRM LEVEL AT 4000 FT.' FO REPLIES; 'NEGATIVE; LEVEL AT 3000 FT AS ASSIGNED.' CTLR SAYS; 'OK; JUST STAY AT 3000 FT.' WE CONTACTED GLASGOW CTLRS BY PHONE ON THE GND; AND AFTER LISTENING TO TAPES; DETERMINED THAT CLRNC WAS FOR ANOTHER ACR AND BOTH OF US ANSWERED THE CLRNC AT EXACTLY THE SAME TIME. ON TAPE IT WAS GARBLED; BUT ENDED WITH AUDIBLE;'...X;' THE LAST NUMBER OF OUR CALL SIGN. CTLR DID NOT ASK FOR CLARIFICATION OF WHO ANSWERED CLRNC; AND WE DID NOT KNOW WE HAD ANSWERED SOMEONE ELSE'S CLRNC. ATC SUPVR ADMITTED LATE RWY CHANGE MAY HAVE ADDED TO CONFUSION. IT CERTAINLY INCREASED OUR WORKLOAD; WHICH WAS ALREADY INCREASED DUE TO THE FOG WX CHANGE; AND WAS QUESTIONABLE WITH SUCH LIGHT WINDS. RESPONSIBILITY LIES WITH BOTH US AND CTLR. WE MISTOOK AND ANSWERED SOMEONE ELSE'S CLRNC (DUE TO OUR HIGH WORKLOAD) AND CTLR FOR NOT CLARIFYING READBACK WHEN IT WAS OBVIOUSLY GARBLED AND ENDED WITH '...X;' NOT '...Y.'