Thanks for your comprehensive reply Rob. The adapter sits flush in the holder just fine - pictures attached. The filter is in the closest slot to the holder.
Perhaps the only thing that you mean with the adapter is that the ridge that protrudes into the holder needs to be deeper, so that it is touching the filter itself? Currently it is not deep enough to do this, at approx 5mm. Surface size is not the issue.
I have just been experimenting in the garden and it doesn't appear that there is any light leaking into 90 second exposures, and when I spoke to someone who worked in this area this morning they seemed to believe that it would not have a detrimental affect as the light is not directly going into the sensor.
I hope all of this makes sense!
Thanks all who have helped so far
Rob seems too have covered all of the points. Ideal there should be no gap.
Watch out for future though. Just because there are no problems in your test shots today doesn't mean it wont happen in the future. It is dependant on filters used, angle and intensity of light, duration of exposure etc.
Glad you got sorted with it, at least you knwo why there is a gap when there shouldn't be now.