There seems to be a small hair or thread in the optical path of my A550. I've partly dismantled it but can't see where I...

I had a similar problem in my a1200 a couple of years ago. I found when looking closely at it that the lens was very slightly off centre. I found by pushing it back to centre it did move slightly. Then I switched it on and the lens then moved more. Switching it on and off several times soon had the lens working correctly. There were some groinchy noises from the lens until it was working correctly. The camera was then fully working correctly and I'm still using the camera now, 10/2017.