SUMPRODUCT - MATCH CALL DETAILS 2 FIND RELEVANT EXTN - URGENT PLS!

[Twiggy]

I have the following data in two separate sheets amidst other rows in two
quite big worksheets:

Date Time Duration Number Dialled Cost
14/01/2010 15:53:24 40 07899123456 £0.035
14/01/2010 15:55:48 609 07899123456 £0.533
14/01/2010 17:04:37 52 07899123456 £0.046
15/01/2010 08:27:49 229 07899123456 £0.200
15/01/2010 08:34:16 11 07899123456 £0.010
15/01/2010 08:38:25 5 07899123456 £0.004
15/01/2010 08:45:37 6 07899123456 £0.005
15/01/201008:46:14 30 07899123456 £0.026

In sheet 2:

Date Time Number Dialled Duration Extn
14/01/2010 15:55:52 07899123456 38 1549
14/01/2010 15:58:28 07899123456 608 1546
14/01/2010 17:06:59 07899123456 51 1549
15/01/2010 08:30:38 07899123456 228 1549
15/01/2010 08:37:15 07899123456 12 1547
15/01/2010 08:41:36 07899123456 4 1547
15/01/2010 08:48:37 07899123456 5 1549
15/01/2010 08:49:11 07899123456 29 1549

I am trying to put in Sheet1, in another column, the relevant extension
number from sheet2 for each call, but because some dates/times are quite
close I cannt get the sumproduct formual to work. Also the times in both
sheets do not match exactly - should I incorporate the duration - but Sheet2
duration is not always lesser than Sheet1 duration (see row 5, sheet2)?

Any help please?

Thank u v much