You should *at least* be available when you say you're going to be

[#1]

I see. So it looks like your router does not respond to pings - which make/model are you using?

[#2]

Quote from: YellowBarley

What I can't run is the broadband quality monitor, with security on or off it just returns 100% packet loss.

Ah - clarity!  If its the "new" 2704n router it (rather unfortunately) does not support ping response on the WAN.  A bit of a hole shot in own foot (by PN) in respect of congestion diagnostics.
I'd stick with the consistency of the majority of speed test results - if they say 15Mbps, then that is what you are getting.
As for streaming lags, then may be there is exchange congestion - something you cannot assess with a router which does not allow inbound ping response, though as I stated before a slow US is not overly helpful. Hopefully Steve can get an angle grinder on that.

[#3]

Long story, short: we're going back to ADSL 2+.
scrookes and I had a long discussion today. He doesn't think fibre is viable to our premises over a 2.5km line length. He also said any further diagnosis will serve only to confirm the situation rather than reveal solutions.
Apparently the root of our problem, as others have stated, is twofold: line length and upstream performance. We're maxing it out each and every day right through until 5am. This is causing the unusable downstream performance.
However, our usage hasn't changed in the time we've been using fibre. So I sincerely hope the further downgrade to ADSL improves things. Our ADSL will be 'uncapped' - which I believe opens up a possible performance gain both up & down.
Thankfully we have an un-utilised phone line to which this can be provisioned, otherwise we'd be looking at likely downtime on top of everything else.
We may then look to go for 2x ADSL connections in an attempt to get the performance we were sold (but which doesn't appear deliverable in one connection) by Plusnet.

[#4]

...or by any ISP on those long BTOR phone lines.
If you want to load balance across two lines, you'll need a completely different set of kit - ADSL modems plus a load balancing router in place of the PlusNET router / modems.

[#5]

Yes of course @Townman my phrasing could have been better - *no one* can deliver fibre at acceptable speed to our property.
Having said that it was a service PN were happy to sell us all the same and whether they should have is debatable. It's wasted a lot of time all round and left us trying to operate a business with a vastly substandard connection for 7 weeks (so far). A simple check at the time of sale would have prevented all that, but also lost them a fibre sale.  To PN's credit they did do that once I highlighted the problem and complained but it's all a bit reactionary and takes *so long* to sort out.
My understanding is that we'll be supplied with the required modems as part of the new ADSL install/s and I had a chat with our tech guy last week who advised we already own (but aren't using) a dual WAN router which should do the trick.