Does anybody know what they should be set to as having problems with backfires on overun.

It backfires when it goes really lean.

Ours is set (from what I remember) to come in when its above 1500rpm and after a 2 sec delay.

You've just got to play about with it a bit, with those settings it only comes on when theres a really obvious over-run. If it triggers too early when your changing throttle often then it causes the popping.
Ours now only pops a little when you go back on the throttle after a period of overrun but were still finalising our VE tables and ignition timing so it could stop popping yet.