In reply to aln:
The answer is of course that it can do absolutely anything in October, from dry heatwave to a solid week of rain, to blizzards, and everything in between - but that it does tend to be unsettled.
You're right to ask about the micro-climate thing, the western peninsulas see a lot less rain than hillly areas inland from them.
In all seriousness, the Met Office have this covered:
https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-actual-and-a...
Have a play with the rainfall and temperature selections, and look at October in past years, you'll see the effect pretty clearly.
If you have accommodation, you'd be unlucky to have a miserable time - there are very likely to be good days, or at least decent windows where you can walk, potter around or boulder. Climbing routes in the hills is unlikely to be great, though (unless winter comes early...)
Oh, and the midges should be gone, at least.