Well I sorted out the issue with the thumbs not displaying....it was a checkbox in Folder/Search options ticked - "Always show icons not thumbnails"!! Easy when you find out the culprit - can't remember ever checking that option but I guess I must've at some point (prolly after one beer too many one night...
But thanks to Marks suggestion, I now have a new default image viewer.....thanks Mark - cracking little prog - and best of all, like Irfanview it's Free
It's deffo worth a donation to the devopers - I always like to show appreciation with a donation for worthy software - and this IS one of those cases.
Thanks again for the pointer