FLC OF A BRITISH HAWKER; HS25 OVERSHOT ASSIGNED ALT IN CLB DUE TO THE AUTOPLT FAILURE TO CAPTURE SET ALT AND THE ALT ALERTER TO PROVIDE AURAL WARNING.

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ATC GAVE 'CLB TO FL230' CLRNC. FL230 WAS SET IN ALT SELECTOR. ACFT WAS ON AUTOPLT AND FMS. AT APPROX FL240 CAPT RECOGNIZED ACFT WAS ABOVE PRESET ALT. HE CHALLENGED COPLT (WHO WAS AT CTLS); 'WHAT WAS ASSIGNED ALT?' AT THIS POINT THERE WAS CONFUSION AS THE LAST ASSIGNED ALT AND IF ALT WAS FL230 ASSIGNED; WHY DID AUTOPLT NOT LEVELOFF AT FL230. AT THIS POINT ZID REQUESTED 'SAY ALT.' CAPT REPLIED FL240 AS ACFT WAS GOING THROUGH FL240. ZID COMMANDED 'DSND TO FL230.' IMMEDIATE DSCNT WAS MADE. NO INCURSION OR EFFECT ON ANY OTHER ACFT WAS MADE. NO OTHER ACFT IN THE AREA. UNKNOWN WHY AUTOPLT DID NOT STOP AT PRESELECT ALT. PLTS DID NOT CATCH MISTAKE BY USING SOP OF CALLING 1000 FT AND 300 FT PRIOR TO LEVELOFF AT ASSIGNED ALT. DID NOT USE GOOD SITUATIONAL AWARENESS IN THAT ALT WAS MISSED AND THERE WAS CONFUSION AS TO LAST ASSIGNED. POSSIBLY TOO MUCH PSYCHOLOGICAL RELIANCE ON AUTOPLT/FMS AND LACK OF MONITORING BY PF AND LACK OF CHALLENGE BY PNF. ADHERENCE TO SOP WOULD HAVE CREATED A REDUNDANCY THAT WOULD HAVE CAUGHT ERROR. ACFT'S AVIONICS ARE RELATIVELY NEW AND HAD NOT HAD ANY PRIOR PROBS IN THIS AREA. POSSIBLY CONTRIBUTING TO A LACK OF VIGILANCE IN THIS AREA.