ALT OVERSHOT BY A B767-300 PIC WHEN THE CREW FAILED TO SET THEIR ALTIMETER TO 29 PT 92 FOR A CLB ABOVE THE 6000 FT TRANSITION ALT NW OF BNN VORTAC; FO.

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ATC FACILITY; LONDON CTL. LOCATION: N OF BOVINGDON VOR. WE DEPARTED LHR VIA THE WOBUN 2F SID. WHILE FLYING N OF BOVINGDON ON A HDG OF 350 DEGS; WE WERE INSTRUCTED BY LONDON CTL TO CLB AND MAINTAIN 7000 FT. WHILE EXECUTING THE CLB WE RECEIVED A TCASII TA IDENTING A DEVELOPING CONFLICT -- AN ACFT 1000 FT ABOVE US. WE REACTED TO THE TA BY LEVELING THE ACFT AND ADVISING LONDON OF BOTH THE TA AND OUR LEVELOFF ACTION. LONDON REPLIED THAT THERE WAS NO CONFLICT; AS THE OTHER ACFT WAS HOLDING AT FL080. AT THAT POINT; WE REALIZED THE SOURCE OF CONFUSION AND RESET OUR ALTIMETERS TO 29.92 AND LEVELED AT FL070. WE BELIEVE THAT LONDON CTL INADVERTENTLY CLRED US TO 7000 FT INSTEAD OF CLRING US TO FL070. I USE THE WORD 'BELIEVE' BECAUSE TO THE BEST OF MY RECOLLECTION; THE LONDON CTLR USED THE TERM '7000 FT' INSTEAD OF THE TERM 'FL070.' A SECOND CONCERN IS THE POSSIBILITY THAT WE GENERATED AN ALTDEV BY OVERSHOOTING FL070 WHEN CLBING TO 7000 FT WITH AN ALTIMETER SETTING OF 29.23. WE BELIEVE THAT WE STOPPED THE CLB AT FL070 AS A RESULT OF THE TA; BUT WE HAVE NO WAY OF BEING CERTAIN.