My timing is set to 6deg BTDC but when I have the timing light on, it intermittently retards by a few degrees. It doesn't cause any problems because it's so slight but I would like to know the cause. 
The Bosch dizzy has been removed to check the roll pin that holds the drive gear in place (all good) and no obvious vacuum leaks. It's a completely standard engine with points ignition. TIA

It could be the bob weights fluctuate from inconsistent vacuum on idle, remove and block vac line and try again, it can also just be the points rubbing on the cam. Points dizzy are very agricultural to today's ignition systems...I bought a Accuspark dizzy from UK and it's was very smooth on idle, no vac line option as motor was lightly tuned

Thanks Marty. Yep the points! Says they should be changed twice a year and I've not done it yet. I'll fit new points and put the light on it to see if that changes anything.