B767-300 CREW HAD ALT OVERSHOOT IN EGLL CLASS B AIRSPACE DUE TO ALTIMETER SETTING ERRORS.

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ON WOBUN2F DEP FROM LHR; WE WERE LEVELING AT 6000 FT WHEN CLRED BY ATC TO CLB TO FL70. ALTHOUGH PREVIOUSLY BRIEFED; NONE OF US CAUGHT THE TRANSITION LEVEL AT 6000 FT. DUE TO THE VERY LOW ALTIMETER SETTING; THE DIFFERENCE BTWN 6000 FT AND FL70 WAS ONLY 400 FT. CLBING TO FL70 WITH THE ALTIMETERS SET AT 993 MILLIBARS CAUSED US TO OVERSHOOT OUR ALT BY APPROX 600 FT. ATC ISSUED A DSCNT CLRNC TO FL70 AND ADVISED US TO CHK; OUR ALTIMETERS FOR 1013 MILLIBARS. THERE WERE NO TA'S OR RA'S DURING THE EXCURSION. WE WERE INFORMED THAT THERE WOULD BE ACTION TAKEN. SUPPLEMENTAL INFO FROM ACN 500039: CORRECTION TO THE ABOVE: THE DEST WAS BOSTON. A CONTRIBUTING FACTOR TO THIS MISTAKE WAS THE LOW ALT WORKLOAD INHERENT WITH TERMINAL OPS. SUPPLEMENTAL INFO FROM ACN 500040: THE FIELD ALTIMETER WAS 988 QNE (29.17 QNH). DURING A BUSY PART OF THE DEP; ACCELERATING TO 290 (CLRED UNRESTR DEP); TURNING ON THE SID; CHANGING RADIO FREQ; AND ANTICIPATING A LARGE ALTIMETER DIFFERENCE CHANGE (29.17 TO 29.92); WE INCORRECTLY SET THE ALTIMETERS TO 29.92 AT 6000 FT WHEN THEY SHOULD HAVE STAYED ON 988 QNE. THIS RESULTED IN A LARGE ALTDEV. A THOROUGH UNDERSTANDING OF THE CORRECT INTL ALTIMETER PROCS WOULD HAVE PREVENTED THIS FROM HAPPENING. KNOW THE DIFFERENT PROCS FOR INTL FLYING.