I think the point about fishing over lunchtime applies mainly to spring fishing, where any rise in temperature in the middle of the day can sometimes bring the fish on. It might also hold true in late autumn if we ever get any properly cold weather, but I wouldn't think that in Sept-Oct in West Wales your river would be cold enough for it to matter.
From my experience of fishing the same beat of the Spey in mid-late September for a number of years, I'd say that the most productive times have been mid-morning (say 10.30-12.30) and evening (5.30-7.30 (or whenever dusk is)). Plenty of people have tried fishing early, but I don't recall anyone having much success.
Other people may wish to put forward their own theories as to why this should be; my feeling is that earlier in the morning the air can be quite cold (I never have much confidence when the air is colder than the water), while in the middle of the day at that time of year the sun can still be fairly bright. Theories aside, and whatever the reasons may be, this is based on a large enough sample, taken over enough years, for me, at least, to have faith in it.
Perhaps the only exception would be if you get a really good spate, when there's a greater chance of catching fish right through the middle of the day.