I finally had a chance to fine tune the barn door last week. A few days with a borrowed 550D, with it's larger review screen and Live View, meant I could do some decent field testing.
First night... and some flawed methodology led me a little astray. I was shooting once a minute, for a few minutes, then checking for drift. There was a small drift, so I did some maths, figured out how far off it was, and adjusted the motor speed. No joy, still getting trails, albeit small ones. I was close, but another session with screwdriver on hand for adjustments on-the-fly and I still couldn't get it right. Ah! Maybe it's not trails, maybe it's wobble - perhaps the motor or mechanism is uneven.
Another test run, this time taking a 2s shot every 10 seconds for a minute. A minute is one cycle on the mechanism. Zoom way in on review, step through the shots, and I could see what was up - the stars would creep ahead, then drop back again. The motor or mechanism wasn't turning at a constant speed. I already knew I hadn't mounted the drive nut onto the main cog quite dead centre (it slipped in the clamp), so today I ripped it off again, and remounted. Fingers crossed that was the issue, the off-centre nut was putting pressure on the drive. Next time we see some stars, I'll get out and see how we're doing!