To echo and add to a few points already made:
Sky haven't "altered something" other than cap your line to 1024kbps to try and give you more stability.
Your line is very long. ADSL technology isn't good with long lines.
Your line condition has deteriorated in the time you've had Sky. This may be the BT line. More probably it is your home wiring.
The drop in noise margin/sync in the evenings is the router attempting to cope with additional induced noise. This is very common in the evenings and is very problematical on poor quality lines such as yours. Induced noise can be many things e.g. environmental (am rf propagation after dusk), fluorescent lighting, other electrical equipment, street lighting etc.
You have an old style BT socket which probably indicates all your wiring is old too. Not having an NTE5 makes it near impossible to isolate your home wiring and determine whether the problem is local or the BT line.
BT say there's no fault. They may be right but that's their standard initial response. They are only worried about voice faults and offer no guarantee/service level for ADSL.
You have three options:
- Invest the time and money to improve your connection. If you can't do it yourself then you'll have to pay BT (expect ~£250 to revamp your home wiring). If successful you'll probably end up with a stable 1 meg connection but there's no certainty that your problem is fixable.
- Get Sky to cap you down to the maximum stable rate (256/512?)
- Decide that ADSL isn't for you. Get cable if available otherwise go back to dial-up.