First step is always to look at the router stats, and see how that is connecting.
If the line has really got worse, then changing provider may result in no difference in service. if the problem is congestion on TalkTalk then of course it may get better if your new provider is less congested.
As for relocating the router and changing filter advice, it is not useless, filters can degrade and which socket the router is using can have an impact. For example if you have got a new mobile phone its mains charger may be creating interference, so moving one or the other might improve speeds.