Hi thanks for your replies.
Where can I get those from please?
I might get the Tamron from LCE as I have been offered £70 for my Nikon lens. Where Castle offered me £60. In LCE I'm getting more reach for my money. Although the switch to stop the lens sliding out is more fiddly on the Tamron.
Are Tamron good for sharpness, or should i spend the extra money on the Sigma?