Out of the two the 120-400mm would be better as it is F5.6 at the 400mm range. You could also consider the Nikon 300mm F4, which cost under £1,000. Just checked the Nikon website and you can use older AIS lenses so, check those out.
Thanks OldBoy for your comments
After lots of hours looking at internet reviews I decided on the 120-400mm , not only for the sports but possibly wildlife with the 1x4 converter attached.
Should be interesting times ahead.
Will try and post some photos for anyone's comments.
Many thanks TJC