Nobody seems to be able to reply to this post - I must have annoyed the timing belt gods!

I hope you don't mind me pasting the guts of your PMs as a sort of manual reply method:

From Voicecoil:
I used AT5 belts (Syncroflex) and pulleys from Beltingonline and got good results - approx +/-10...15um final accuracy. There are different tooth profiles available for belts, the main ones being trapezoidal (lowest backlash) and trapezoidal (best power transmission), I'm wondering from what you said about the teeth not seeming to reach the bottom that maybe your belt is a different profile from the pulleys? Hope this is of some help, Trevor

From m_c:
Have you tried knocking a flange of a pulley, so you can see how well/poorly the teeth mesh?
They should be a pretty tight fit without any movement.

I'll have another look for alternative versions but after a first look I couldn't find an equivalent small pulley of a similar diameter to the 16t one. However, I have a flangeless pulley on the motor, so may be able to look more closely at how the belt and pulley mesh. I'll look at beltingonline as an alternative supplier, as I can't help thinking this is a quality issue.

Thanks for the replies!