@tubaman - One could only hope with TalkTalk... Expecting results for their consumer products, as steenamroo has experienced, is a lost cause.
@steenamaroo, I do actually have a suggestion for you...
If you're still in contract (which it seems you will be because you switched recently) with TalkTalk, you should request Minimum Guaranteed Access Line Speed (MGALS) from them. The MGALS should specify the 10th percentile speed for similar customers on the same service, at which point they must act to resolve this problem by raising it as fault or let you leave contract free if they have raised it to OR and they have found no fault and the line is at its max capability.
You have done your part, as a customer, by replacing their awful kit, changing filters, etc. Your line is clean and certainly capable of the full speeds as you have previously experienced. There's nothing really more that an ISP could expect you to do.
ISPs should be able to do a remote DLM reset (limited to 1000 circuits a day), though I do not know if TalkTalk have access to or use this yet. I would even consider calling their faults department and state that your line is underperforming because of a "fault" with their own TalkTalk router hardware and that you need them to reset the circuit's DLM after replacing it. As horrible as calling them sounds, you need to try a different avenue because the OCE's on the forum are not giving you straight answers or even providing real support in my opinion.