Andibevan wrote...
As far as I understand the standard Bin2Dec and Dec2Bin functions can
only
handle 10 character binary numbers.

Does anyone have any pointers on how to handle binary numbers greater
than
this?

If I have completely mis-understood how excel handles binary numbers
feel
free to let me know.

You've been given a udf, which is flexible enough to handle many
radices. A quicker alternative would be downloading and installing
Laurent Longre's MOREFUNC.XLL add-in, available from

http://longre.free.fr/english

which includes a function named CHBASE that does the same thing.

If all you need to work with are binary numbers, you could forgo both
udfs and add-ins and use divide-and-conquer.

A1: (decimal number)
7654321

A2: (binary string with leading zeros)
=DEC2BIN(INT(A1/2^24),8)&DEC2BIN(INT(MOD(A1,2^24)/2^16),8)
&DEC2BIN(INT(MOD(A1,2^16)/2^8),8)&DEC2BIN(MOD(A1,2^8),8)

A3: (binary string without leading zeros)
=MID(DEC2BIN(INT(A1/2^24),8)&DEC2BIN(INT(MOD(A1,2^24)/2^16),8)
&DEC2BIN(INT(MOD(A1,2^16)/2^8),8)&DEC2BIN(MOD(A1,2^8),8),
INT(33-LOG(A1,2)),32)

A4: (decimal number derived from A2 - would be the same for A3)
=SUMPRODUCT(--MID(RIGHT(REPT("0",32)&A2,32),ROW(INDIRECT("1:32") ),1),
2^(32-ROW(INDIRECT("1:32"))))