Thank you for writing back with that information. If the problem is with regards ***** ***** internet speed, the only way you can test it better, yes, even from your end -- not just from talktalk, is to isolate all other devices, and just connect a computer through the provided ISP router, then test the browsing speed using online test tool like speedtest.net. If your generated speed in the test is too far below the subscribed speed, then there is an issue with your account or internet line. Your ISP (Talktalk) needs to check it from their end or might need to deploy a technician onsite to fix the problem.
However, if the speed of the connection is within your subscribed bandwidth in the isolation test that I described, and that the problem only happens when all of your devices are connected, then this means that there is no problem with your internet speed. Those other devices are just sharing the subscribed bandwidth/speed and in effect, your connection will become slower.
Now, regarding that TV box problem, if the led indicators are not lit especially the power led indicator, it means that the device is faulty and it needs to be replaced. Please contact your ISP for further online and/or onsite testing and possible replacement.
Thank you and I hope that helped.