Thanks for the responses all. To clarify a few things:
* the derailleur pulls the chain off the rings entirely on both the small and big ring. It can definitely reach the big one, and beyond.
* the bottom bracket is not from the old frame. From memory it's a 119mm bb, I did some research and thought that was most likely to fit this crankset. If people think there's too much clearance then maybe I need a shorter one after all.
* there's way more unscrewability on those screws. I can unscrew them entirely, but it changes nothing unfortunately.
*if you think it's dirty now, it was filthy beyond your wildest nightmares before.
I didn't know you could see the screws inside the mechanism if you looked at it from the side on, will post a photo of that when I get home!