First, I'll disabuse you of a couple of false notions:
1) Being signed up with Samknows might give you a lot of speed data, but you don't have a wealth of line metrics.
Speed tests give you, at best, a hint that a problem exists. They do nothing to tell you what the cause of the problem is.
Detailed line metrics would help there, but the regular Openreach modem hides those. The best that can be done with a BT-Wholesale-based line is to perform a BTW speedtest, and extract the IP profile, from which a sync speed can be calculated.
Unfortunately, even this piece of information is unavailable to TT lines.
2) FTTC speeds start high, and pretty much only ever get lower.
There are multiple reasons, even on a line that isn't faulty. Line faults can add an extra dimension to speed reduction, and intermittent line faults can both cause speed reductions *and* be incredibly tricky to fix.
Reason 1: DLM usually watches for 2 days before intervening. If your line has a high error rate, or even high packet loss, then DLM can intervene. Speeds can be reduced by anywhere between 10% and 30%.
A visiting engineer can order DLM to be reset, at which point it monitors for another 2 days before intervening again.
Reason 2: The major cause of noise and interference to an FTTC line is other FTTC subscribers. This causes speed reductions as take-up increases.
The problem is that the speed reductions seen are, effectively, random compared to the take-up, so the extra reduction can happen with just one or two other subscribers out of 100.
Speed reductions here can be even higher.
Reason 3: You might have a faulty line.
So... what does this mean?
When you see a speed of 70 on an engineer's JDSU, you can't expect to keep it. You need to see the error rate too, to have an idea of whether the speed will be kept.
These are the line metrics you need from your modem: Sync speed, CRC error rate/count, FEC error rate/count, and the FEC/interleaving settings ordered by DLM.
Is any of this available from the HG533 router? I don't know this router very well (or the TT service) ... is the router acting as a modem?
If DLM has intervened, it will have set a line profile for the modems to follow. I've seen Plusnet get hold of this information from Openreach... can TT get it too?