Hi Graham,
some time ago I decided to give the mirror in my 12" LB a wash, and was horrified to find all manner of speckles all over it. I hadn't realised just how bad it had become until I took it out and had a close look - much the same as yours as far as I can see.
I asked here on the forum, and we more or less came to the conclusion that it was the result of a year or so's lack of care after a night's viewing. Meaning that, after use, I have always just stored the scope in the shed with the ota pointing vertically. There has always been a net curtain type cover over it. But naturally, any condensation that has formed has just sat there drying overnight, and depositing muck on the mirror surface as it does so.
I did still clean the mirror as best I could, but it does still look really quite yuk. The good news is, it doesn't seem to degrade the image quality hardly at all. Now, after use, I rest the ota horizontally in it's mount.
Hope this helps
Cheers,