Is this normal / acceptable, or a problem that can be fixed?

[#1]

Hello there. From the information provided here, it sounds like the fault is either congestion on the BT network, or a wireless issue as pointed out by Andrue. Whilst our faults team test your broadband and get back to you via the ticket raised, I'd recommend checking out this guide on wireless connectivity to see if it resolves your problem.
http://www.plus.net/support/broadband/wireless_broadband/wireless_signal.shtml

[#2]

Quote from: Andrue

Why is it not possible? All you need is an Ethernet cable to connect your laptop to the router.

Well aware of that but SWMBO would have thrown something at me if I'd played about anymore at that point 
For reference one actually needs an ethernet connection to initially activate Hub One.
Router actually has line of sight connection through open hall door to laptop.
Reason I'm ruling out a wi-fi problem is that the only thing that's changed this year is switch, on Jan 5th, from using HH4 ( purchased 2 years ago) with adsl to using Hub One with fibre.
As usual speed is back to normal this morning http://www.thinkbroadband.com/speedtest/results.html?id=145621832771829447764
Having had broadband for 14 years, and used wi-fi for at least 8 years, I find it hard to accept that wi-fi problem could cause such a massive variation in download speeds

[#3]

Have you confirmed that none of your neighbours has made any change to their WiFi setup?

[#4]

Problem has existed since I got fibre on Jan 5th
Running inSSIDer nothing seems to have changed.
Neighbour other side of wall in semi has HH5 running and that's been there over 2 years. He's not a tinkerer.
Despite silly speeds I was still running Amazon Prime, using Roku Stick, with no buffering after 9pm yesterday.
Am thinking of getting a spare router for safety reasons so may have a play with that in due course.

[#5]

Just had phone call from Plusnet and seems that it is a capacity problem so am being switched to new equipment to see if that improves things 

[#6]

Don't know exactly what was moved but it worked so thank you Plusnet 
No slowing down at all during peak times yesterday evening - ran regular tests using phone ( wi-fi not 4G  ;)) and got speeds up to 35Mbps
All I can see is that internet reconnected 32 hours ago - dsl connection not affected and did not go down.

[#7]

Glad to hear things have improved.

[#8]

Could you post a tracert to ntp.plus.net please.

[#9]

Microsoft Windows [Version 10.0.10586]
(c) 2015 Microsoft Corporation. All rights reserved.
C:\Users\John>Tracert ntp.plus.net
Tracing route to ntp.plus.net [212.159.13.49]
over a maximum of 30 hops:
  1    10 ms    2 ms    2 ms  dsldevice.lan [192.168.1.254]
  2    10 ms    14 ms    14 ms  lo0-central10.ptw-ag02.plus.net [195.166.128.196]
  3    15 ms    15 ms    14 ms  link-a-central10.ptw-gw01.plus.net [212.159.2.148]
  4    15 ms    14 ms    15 ms  xe-0-2-0.ptw-cr01.plus.net [212.159.0.248]
  5    13 ms    15 ms    13 ms  ae2.ptw-cr02.plus.net [195.166.129.5]
  6    11 ms    15 ms    14 ms  po2.ptw-gw02.plus.net [195.166.129.39]
  7    12 ms    *        *    te2-1-4.ptw-ssw01.plus.net [195.166.129.157]
  8    9 ms    12 ms    14 ms  cdns01.plus.net [212.159.13.49]
Trace complete.

[#10]

Thanks
If you do see issues again keep an eye on that (or the gateway checker).