Auto-filter Macro linked to drop down box selection

Hi - Just joined the forum after finding it a useful source for several months now.

I have a sixteen column table of data (called 'Detail Data') and summary on a second tab. On this summary tab I've added three drop down boxes for the user to select from, then a Macro button that they can click which filters the detail data according to the three drop down criteria.

Problem is... I would like the macro to not use the drop down criteria if it equals blank. For instance, if somebody only wanted to filter by two of the three criteria. Otherwise the blank cell is selected as filter criteria and results in no detail data being displayed.
I'm very much a novice in the VBA dept (at the recording, then faffing about trying to edit small sections kind of level) - any help would be much appreciated.

Windows XP
Excel 2010

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JapanDave - You're an absolute star.

Thanks very much. I'll study this code for future use.

Gregg