AN EMB145 FLC START AN ILS APCH; SWITCHING TO TWR FREQ WHEN THEY REALIZE THAT THEY HAD BEEN CLRED FOR THE ILS APCH TO RWY 14R WHILE 3 MI BEHIND A B747 AT ORD; IL.

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TRACKING INBOUND LOC RWY 14R ORD; WE WERE BEING STEPPED DOWN FROM 7000 FT TO 5000 FT FOLLOWING A B747. WE WERE ABOVE GS. ATC HAD NOT ISSUED AN APCH CLRNC BECAUSE OF SPACING BEHIND THE B747. WE HAD BEEN SLOWED FROM 210 KTS; TO 170 KTS THEN TO 160 KTS; THEN CLRED DOWN TO 2500 FT. I WENT TO LNDG CONFIGN IN ORDER TO SLOW DOWN AND MAINTAIN A DSCNT PROFILE THAT WOULD BRING ME DOWN TO THE MARKER XING ALT AND ESTABLISH A STABILIZED PROFILE FROM THE MARKER INBOUND. WE HAD VISUAL WITH THE B747 AND THE ARPT. APCH CTL HAD NOT ISSUED AN APCH CLRNC. AT THE MARKER WE SWITCHED OVER TO THE CTL TWR AS IS NORMAL PROC AT ORD. I WAS IN A MINDSET OF CONTINUING TO LAND AND COMPLETING THE BEFORE LNDG CHKLIST. AT THE MARKER AND GS INTERCEPT I CONTINUED DOWN THE GLIDE PATH AND AT 2300 FT REALIZED WE DID NOT HAVE AN APCH CLRNC AND LEVELED OFF. THE FO CONTACTED THE TWR AND WE WERE GIVING THE OPTION OF A GAR OR A VISUAL BEHIND THE B747. WE HAD ADEQUATE SPACING AND VISUAL ON THE ACFT AHEAD AND LANDED WITHOUT INCIDENT. THERE WERE SEVERAL FACTORS INVOLVED LEADING UP TO THIS SIT. WE WERE BEING HELD HIGH WHILE THE B747 WAS VECTORED ON THE LOC IN FRONT OF US. THE B747 REDUCED HIS SPD UNEXPECTANTLY AND WE HAD TO FULLY CONFIGURE IN ORDER TO SLOW DOWN AND DSND FURTHER OUT THAN NORMAL; WHICH LED TO A VERY BUSY COCKPIT ENVIRONMENT.