[deadsimple (Ash)]

Thanks for the replies.

I ended up getting most, but not all of the residue off that little section without touching/cleaning the whole mirror - it was incredibly stubborn. It's not that visible when I look at my reflection in the mirror under decent lighting, but I can see some minor bluey-white residue when looking at the mirror at an angle under a strong white light. I guess the main thing that stands out is that most of the mirror is dusty and this 1/5th of the mirror is dustless but with some residue.

Would that be good enough or can it really affect observing and astrophotography?

(I've read in a couple of places that it will always look dirty after a clean if you do the torch test, and just to do it by eye near good light instead - if it looks fine then that's good enough)