It's worse than that, local camera shop agreed that it was probably an electronic fault with the lens, but said it was beyond economic repair! It's 3 years old and thus out of guarantee. Nikon charge nearly £90 per hour +VAT to even look at it so a new Sigma lens with almost the same spec (on special offer at £150) would probably be cheaper. .
Now if the body had been more modern I might well have bought this, but the D100 is now some 8-9 years old and very much outclassed in many respects by even a small compact. So I passed. I can't afford a new body equivalent to what the D100 was in its heyday and it didn't make economic sense to buy a new lens to go with a down level body.