My 2 pence worth:
SMART WI-FI
I can confirm that SMART WI-FI (auto channel swap) is poor in my experience, every time I've tested such a feature across various routers on various connections (not just my own - which I've tested many routers on) it seems to always pick one that doesn't appear optimal? (i.e. switching to an active channel when there are clear channels on 1, 6 or 11 on the 2.4Ghz spectrum). I know someone has already mentioned that the router/hub may have more information when choosing a channel, but the only thing I can think of is RF Spectrum Analysis and I may be wrong but aren't these types of devices expensive? and therefore unlikely to be installed in a cheap ISP router?
PLUSNET HUB ONE
I will also add (even though its probably irrelevant at this point) that the Plusnet Hub One is absolutely DIRE in my honest opinion, I had 2 from Plusnet as the first would drop its throughput from 36Mb to 15-25Mb every 4-5 days and only a reboot would fix it, the 2nd Plusnet Hub One done the exact same.. I resolved this by using my own.
THROTTLING
Your 100% not being "throttled" by your ISP, there is no way for an ISP to manipulate what goes on in your internal network, also what gain is there for Plusnet to pump 65Mb down to the router and then to throttle it and cause you issues at your end? it just doesn't make sense and can not and does not happen.
NTE5C+Mk4 SSFP
There are loads of bad reviews about these, I myself just 3 days ago tested various SSFP's/setups including the one you have even after hearing this as I wanted to find out for myself, I had loss of speed (2MB) and errors went up 10 fold (and strange readings from DSL-stats), you would have been better leaving the NTE5A and Mk2 SSFP in to be honest, its looks nice, but is cheaply made (no circuit boards, just metal strips) in comparison to the older one
CONCLUSION
After reading your thread I have to say that it sounds like its an internal configuration issue or the device(s) your testing have issues. You mention having switches and routers etc, maybe if you could supply more information about your setup and the devices connecting to it?
What routers/switches do you have?
How are they connected?
How are they configured?
Same again for devices.
- sometimes a mockup drawing in paint or a photo can help.