Thanks Don, but I don't think this is the case - the lens is still autofocussing fine, but because the switch has stuck it won't manually focus (same happened before). I forgot to mention (perhaps critically) that this seems to happen when I have used a teleconverter.

I have had an ist DS for about six months - after a few months of use having set the camera to focus manually, the a/m focus switch jammed on the body (to manual) yet the lens was focussing automatically. The body went back to Pentax and they replaced it - a brand new body. Great - but after a week of use, exactly the same problem has happened again - my Sigma 170-500 having being focussed manually has caused the switch to jam again!!! Seems like a problem with what I'm doing or the lens - but how can a lens cause a mechanical fault on the body - any help??