Sigma 170 - 500 lens

Location : Peterborough

Amongst many lens I use I have a rather old, but still very good, Sigma 170-500 APO and I use it on Canon 400D. The lens seems to work OK at F5.6, with good aperture / shutter read readings and well exposed images, however as you stop down, although the camera is give good reading ie the shutter speed is reducing in proportion to the increased aperture, the images become over exposed until at F36 they are totally over exposed. My other lens are still working OK with the camera. My assumption is that the diaphragm is not stopping down as pressing the depth of field preview button has no affect. Is there any manual way to 'reset' or check that he diaphragm is not jammed fully open ? I have contacted Sigma UK and a service starts at £90 plus any repair parts, does anybody know of alternative (cheaper) repairers.

Thanks Paul