crwdns2893792:0crwdne2893792:0 Philip Le Riche ,
Whatever it is, the fact that it goes away (progressivle, I presume?) as you increase the f stop indicates it’s not in the focal plane (where the sensor is). I’m more familiar with compact digital cameras, but these have a focussing lens which sits just in front of the sensor and is moved forwards or backwards in order to focus. This might be in the right position to cause the spot.