Just turned 5pm and my speed has instantly dropped again.
Interesting how BT and Thinkbroadband speedtest says I'm getting the full speed - 1400kbps for my connection.
DSLZone, Speakeasy and speedtest.net say I'm getting 200-400kbps and as web browsing is very slow now, I'm inclined to believe the latter.
If your router is syncing at 1400 bps but actual throughput is much lower after five o'clock then it is probably contention kicking in. Peak time usually starts at about six in the evening but that's just an average of the whole country. In you case maybe you have a lot of school kids locally who are downloading or maybe searching to do their home work? Alternatively some heavy local users who start downloading at five to avoid peak time at six. Remember you could be sharing bandwidth with up to 50 other users. If you want lower contention ratios then BT BB Business has much lower contention ratios.