Trying to troubleshoot connection problem

So, for the last month or so my internet connection has both dropped in speed and reliability. I have Sky Superfast Broadband. My actual download speed has dropped by almost half (from 9mb/s to 5mb/s), and my internet connection drops out 2/3 times a day. Both problems effect wireless and ethernet. I do not have many devices connected to the network and I am not a heavy user.

I have had two Openreach engineers out who say there is nothing wrong outside the property. A Sky Expert said that there is a fault inside the property but because I am on a rolling contract I cannot have an engineer visit without paying £65 or signing a new contract.

I have done some troubleshooting myself. I have connected the master socket directly to the cable coming into the house, I have also used a micro filter instead of the master socket. Neither has helped. The only internal factor I have not changed is the router.

With the Sky Expert saying there is an internal fault, what else can I try?

Thanks.

Do you have any telephone extension sockets?

If so, do you have a filtered faceplate at your mast socket? Something like this.

Look at Interference Video under Guides in Thinkbroadband.
Wireless phones can be a problem. If you have unplug this first.
First switch off all electrical power. Unplug all sockets leaving just Router on mains. Boot Router and check stats.
If problem resolved, gradually switch on rest of sockets checking router stats. Or follow the guide with AM/FM radio.