Our village exchange has recently had work done and a few days ago we found out our Broadband is now on the 21CN.

The original 8Mbps Connection (5-6Mbps avrg speed) is now up to a 21Mbps Connection with a IP Profile of 19.3 Mbps and a (14-17Mbps avrg)

All has been good until a couple of nights ago when we started getting slowdowns after 5-6pm. Streaming video and other things have become slow as if we are being throttled.
This never used to happen before. On the old connection we got 5-6Mbps avrg all the time day or night. Speed Test Results sometimes come back between 2-4Mbps! BT Wholesale Diagnostics now also come back slow where as they go up to 17Mbps+ during the day.

If We are Unlimited BB on 21CN then why are we now slower in evenings than we ever have been?

That does sound like congestion on the backhaul which is unusual on 21CN.

Attempt to get it reported to BT (if you can make them understand the difference between throughput and sync speed that is). Hopefully a job will be raised to increase capacity.

These things are monitored anyway so it should happen anyway if you just leave it.

Still no change. Around 17 meg during the day and a drop below 4 from about 5-6pm until 10-11pm. The exchange only serves 1000 residential properties too. surely 21CN can cope with that?!

"Unlimited Broadband" this is not. Does BT have any port of call to report things like this or do I just ring them?

You can report a fault on their website yes. Or call them.

Your exchange will just be the hub site if it's only small, there will be the main equipment in the larger exchange nearby. Could be the fibre between your exchange and the other exchange or it could be a capacity issue at the big exchange.