Be careful Shazz, I read somewhere that wood filler in the vacant fret slots compresses too much under string tension and the fingerboard acts like its been kerfed. Apparently filling the slots with slivers of veneer prevents this from happening. I am an amateur and have no experience at all in any of this stuff, so it might be worth getting some advice from our resident experts before you go too far down that track.
Just a thought mate.
rob