Hi Herbie,
I also have a Skywatcher 150/750 and found getting that secondary mirror right could be a bit of a pain. If you wind the focus tube out, that black rectangle on the right of your picture should disappear from view, which may have an effect on the blob in the middle. Not sure though.
I found that when I reached in and checked the small mirror that reflects the image into the viewfinder that it was quite wobbly and that getting it exactly balanced and lined up, and then tightened properly, using the three allen key screws in the centre mounting block was quite awkward. The required adjustments were pretty fine.
My only tool was a small pinhole drilled in the centre of a plastic cap that fitted where the eyepiece normally goes. But it was enough. My only precaution was to do the adjusting with the tube facing down so that if I dropped the allen key it would fall out of the tube and not down onto the big mirror.
Sorry that I can't be of more help, but the secondary mirror is held in place by a simple but pretty crude system. Initially I made it a lot worse before I found the right touch and adjustments to make it better.
If in doubt, it might be better to get a more experienced person to assist the first time you do it.
Best of luck with it.
