Hi Bazza, welcome to the forum, I have one of the Sima 150-500 OS lenses and for birds and wildlife, as well as aircraft it's outstanding. I got mine from
www.panamoz.com and can be had for around £600 new. I had a Canon 100-400 IS L as well, unfortunately one of them had to go, and when I compared it with the Sigma I decided to keep the Sigma as I rated it so highly. Here's a link to panamoz if you're interested
http://panamoz.com/index.php/hot-de...m-for-canon-digital-and-film-slr-cameras.html