PLEASE CLICK ON IMAGE TO ENLARGE
(it's a mobile phone image though so quality isn't great)
Closer inspection reveals that the rear plastic section of the lens (the bit that attaches to the camera) is held onto the main part of the lens by 3 very small screws which go through weak plastic fittings. Since the entire weight of this fairly heavy lens rests on these 3 plastic fittings, I'm actually surprised that this hasn't happened before. Any knocks to the lens are highly likely to break these fittings and I suspect they were already cracked and weakened from 6 years of heavy use. When I took off the lens yesterday it proved to be the final straw.
I've sent it to Sigma to be repaired and I'm hoping this will be reasonably straightforward. I imagine they will replace the rear section although repairing the electrical contacts (two of which snapped) might be more difficult.
I have always been a huge fan of this lens. The image quality is excellent and, until yesterday, I always thought it was fairly robust. However, when I do get the lens back I will certainly be treating it more carefully in future as this does strike me as a real design weakness.
UPDATE: Sigma repaired the lens for a charge of £98 and returned it to me within approximately 3 weeks.