Thanks for that, Wavytone! I thought that it must be something from the refractor. Next time, I shall use my Lightbridge - as you suggested - was a bit lazy last night as I have to get the sack trolley out with ropes, to move the Lightbridge
Incidentially, when I used a 17mm EP I did see the blue fringing around the Moon's edge, which disappeared when I put on the lens cap and just removed the smaller central cap!