Just found a weblink for the Lumicon Prom filter which states its a 1.5A bandwidth and MUST be used in an f20 and above system with an ERF.
In fairness to the earlier testing, my system was f10 not f20.
I'll stop the aperture down to 45mm, use the C-ERF and give it another go.
http://www.handsonoptics.com/slr_lumicon.html