It's probably not the best solution but the way you could do this is by the Bump add-on. It'll silently bump the thread without you needing to respond to it. Not sure this is the best solution though but it will mark it as unread once bumped. http://xenforo.com/community/resources/bump-a-thread.446/
Thanks Shelley and Lycaon. The Bookmarks one sounds good. The problem is now that I have to go trawling though the forums looking for stuff I read without answering at the time and I am missing some of them. It was the same in my last forum that I just moved from (IPB). Off to look at Bookmarks. Thanks.
I can tell you Syndol author of the Bookmarks add-on is a great guy and you can bet he will constantly improve this add-on and cater any suggestions you may have for improvements. I've never regretted making the purchase and use it for exactly some of the reasons you have wanted it for. Good to hear your having positive results/experiences with it.