Not been able to log on for a few days, thanks for your help guys!
Andy – cheers, what you says makes a lot of sense, I got the impression I wasn't using the point filter for its intended purpose. 15000 points might be a lot too many, and that was only a 4 hour ride! I'd seen the other mechanism in the Garmin GPS transfer option, but obviously I couldn't use that. However, I've checked out GPSBabel and got it to do exactly what I want, so Tracklogs is running smoothly again :-)
Kojaklollipop – thanks for the offer of the firmware modification, I probably don't need it now as GPSBabel has solved the problem. Curious to know how you managed to get your point recording down to once every 4 seconds though – as far as I can tell, simply switching Advanced Power Saving on should do it, but when I tried it I still got one point per second. Anything else I need to do?
Timmys – thanks for the info, I'd seen it before but never noticed the filtering.