It looks like focusing was the issue with this image! I checked the individual shots and all but the very first test shot are slightly out of focus. After the first test shot I tested an ISO of 800 and must have put a tad too much pressure when pressing the buttons on the DSLR. This in turn appears to have slightly moved the focuser - enough to be only visible on close inspection!!
2 hours later and I wasn't the wiser - aaagghhh.... a valuable lesson learnt here ladies and gentlemen!
Mario