So what is the evening attenuation value flipping between?

Have seen some obsess about 0.2dB changes which given reliability of the calculations/software is not significant.

I've been going by realtime impressions.
Such as BBC tv on iPlayer takes less that 10 seconds to come up (begins streaming) anew through most of the day, in the evenings it takes minutes, often 5 even 10.
This the same using Windows PC, Linux, and Mac.

I don't see the link between varying attenuation and the time for iPlayer to stream

This sounds more like another issue