Will this lens auto focus on my camera?

This lens: http://www.jessops.com/online.store/products/73537/show.html
i have a nikon d3100 which doesn't have an internal AF motor. I'm under the impression it will AF but it is worryingly low price…

Sigma is one of the better make for aftermarket brands. It should post no trouble for your D3100.

It will auto focus i do believe.

Yes it will. That is about the normal price I've seen around for it. You might be disappointed in the image quality if you are a serious photographer.

Personally I don't trust 3rd party lenses because of a bad experience I had.

These lenses are lower priced for two main reasons, quality of gear and of the glass. You can't get the quality of a Nikon lens at the cheaper price of a 3rd party lens… It won't happen.

The Sigma 70-300mm will AF on your D3100, the D40/D40x were the prior models to your D3100 that also lacked an AF motor. The Sigma isn't a fantastic lens, so don't expect it to be sharp past 220mm or so, but it is perfectly adequate at this price. The Sigma 70-300mm APO version is sharper, but more expensive of course. The price is correct for the non-APO version and Jessops are reputable so you will not have any problems there.

Why not go into your local Jessops and try it on your camera, I do all the time.

JimA and his nonsense about third-party lenses never ceases to amuse me…