thank you tt2 - that is pretty much what I did in the end. In most cases if I select the option to view the album in the iTunes Store it takes me direct to the album (which makes it a nonsense that it can't find the artwork!). Anyway, once there I click on the album and when it takes me to the next level with a slightly larger image of the cover I do a "shift+cmd+4" and when the crosshairs come up I select the outline of the cover. This then saves to my desktop so I open it and export to my hard drive, renaming as necessary. Back to iTunes and if you go to "Album Info" and click on the Artwork tab, you can click on the button at the bottom to add artwork, then I just select the file I saved and voila!
Apologies, I am sure you are bright enough to know all that but I just explained in case someone else had the same issue and came across my post :) Much appreciate your feedback - thank you