download a clip and watch same then surf elsewhere or even switch off the machine ,then return to youtube a few hours later click on the same clip and it loads up instantly,so I recon it must be stored somewhere on my hard disk
Correct, it is.
Everything you look at, listen to or watch on any web page is stored in your internet cache. Its usually a hidden directory so you need to set your folder options to view hidden system files and folders.
Once you've done that Right click on the content you want to copy and make a note of the file name, do a search for that file name, all files and folders. When it pops up copy it to another folder of your choice.
this works for all content and you don't need any fancy program, if you can hear it, watch it or look at it, its already on your hard drive.