OK thanks for all the replies guys, that sudden thought has enabled me to solve this.
I copied a hot beam cue into a spare slot int he workspace (so I'd remember to delete it straight after) and reassigned it to Atmospherics (i was behind the projector) then displayed it. Nearly all the tails disappeared when in the size controlled zone - so it was the zone sizing at fault. (Tried with another cue as well to verify). (Obviously when in Atmospherics it shrank visibly in size).
The final bit of tail I found could be got rid of by reducing the scan speed from 30K to 25K which is what Kvant now say the scanners are rated for.
Al.