[johnnyrod]

Well I took the Certo rather further and took out the shutter blades and rings. I was frankly crapping myself, particularly when I found that a couple of springs were leaning against the inside of the casing I'd just removed, but it's a well-engineered piece of kit and went fine in the end. Swabbed a fair bit of muck out of there so worth it. Just have to refit the shutter to the camera, give the body a bit of a clean, and check the focus. Phew. Hopefully it'll sort out the faster speeds, but it's as good as it gets in any case.