Fibre Plus download speed dropped by about 20 megabits. No obvious reason. DLM possible suspect?

Hi Talktalk team, I am having a small issue with my broadband and wonder if you can help. I have been on your "Fibre Large" plan for quite some time now, since around November 2015, and have always achieved 75+ mbps down (and 19 up). Note: we live practically right next to the fibre cabinet, hence the 75+ capabilities.

Overall the service has been fantastic, although it does drop when our landline receives a call for some odd reason (But I'll talk about that another time!). I'm writing here because since a few months ago, our wifi speed has dropped to about 55mbps (Which is still fine, hence leaving it so long to write this!), and I always assumed it was just my router's wireless config going weird (I use your Talktalk-provided router as a modem that bridges everything through to our dd-wrt loaded Netgear R7000 "Nighthawk" router which has improved wifi performance, as I'm sure you know is very common practice).

Back to the issue - After fiddling with wifi settings for a while, but with no luck with raising our 802.11ac speed above 55mbps like in the past, I only today connected my laptop via ethernet, assuming to achieve 75+, only to find that our wired speed is also 55 now. This explained a lot.. Image may be NSFW.
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Now, if you look in my post history you can see I started a thread back in 2011 for a similar issue back when I was on normal ADSL+ broadband, where it turned out DLM knocked my speed down and you magic people fixed the issues for me by moving me to a faster profile. I have done quick research before writing this to find that DLM is apparently still present (although differently configured?) for fibre connections, so I'm thinking this may have detected too many connection errors and knocked my speed down for the sake of reliability? If this is the case (hopefully) then I can assure you we had zero connection or packet loss issues on 75+ speeds, and I'd really appreciate if someone here can undo said change, and disable DLM too (My request to disable was denied when I asked in 2011 but I have seen your team disable DLM for others many times so thought I'll ask again now Image may be NSFW.
Clik here to view.).

I realize for common-procedure reasons, you may want us to test speeds when using the Talktalk router as the primary gateway, before making any line changes... but please appreciate that this is an expensive router we are using and is certainly not the cause of this issue, as it is wired (through the modem) right to the master socket, so no reason for such a loss in speed here!

p.s exchange code: SSBAN

Many thanks in advance, Talktalk team Image may be NSFW.
Clik here to view.