Using GPS Track Data in Aperture

I'm trying to sort out my holiday photos in Aperture and I've got a problem...

My camera has a GPS so it location stamps all pictures as long as it's got a recent fix. It also records GPS tracks for the duration of the shoot (say 1 day).

75% of my shots are properly GPS tagged, what I want to do is use the track data to complete the tagging on the remaining 25% of the shots, as even as there was no GPS fix at the time when these photos were taken, the track should still allow a midway point between the last 2 known fixes to be used. As the timestamps on my photos correspond to the timestamps on my GPS tracks, I though this would be easy to achieve in Aperture by importing the GPS tracks and automatically adding the photos to it, but it would seem not. Aperture doesn't seem to let me do a simple timestamp-matched fix as it seems to disregard any correlation between the photo timestamps and the GPS timestamps. Instead it seems I have to manually drag a single reference picture to the correct point on the track, and it will tag that and all other photos in the same project with location based on their timestamp offset. This isn't going to work for me as I can't manually place the reference picture accurately enough to get the +0 minutes position, partly due to the limited zoom/resolution on the map (+/- 5 mins is as close as I can do it manually), and partly because quite a lot of my track crossed back over itself, so it's virtually impossible to click on the right leg of the track.

I'm sure there must be a better (preferably free!) way of automatically matching photos to a GPS track with a synchronised time stamp... anyone, please?