I've tried modifying the formula you provided after changing the A, B, C to the actual ranges and am getting the same result. Do you see any syntax errors that may be causing the problem ?

=SUMPRODUCT(--(TEXT('Jan ''07'!$A$2:$A$750,"dd/mm/yy")="01/01/07")*(--('Jan ''07'!$E$2:$E$750="London")*(TEXT('Jan ''07'!$J$2:$J$750,"hh:mm"))))