Thanks for the response. I am using a Nighthawk D7000. I have just swapped from a Asus DSL-AC68u and got another 2-3Mb!
SNR has always been a problem out here. I have over a Km of old O/H to contend with. I have had numerous BT folks test the line but they all say it's perfect! I suppose the low CRCs also suggest that there is no fault as such. Very frustrating when the "physics" suggest that I could get 15-20Mb. I see what you say about the timing though, it does look as though there is an early-hours attack! We are in the middle of nowhere with no street lights so I guess I'll never know what causes it.
Cheers