Poor quality streaming TV

[#1]

I moved to Plusnet unlimited fibre just over 2 weeks ago (3rd November).
The primary reason for choosing a fibre connection was to improve the quality of streaming TV (iPlayer, etc), but the viewing experience doesn't seem to have improved at all. I get intermittent buffering/pixellation and occasionally the player seems to hang - the spinning circle doesn't disappear until I reload the service. On a few occasions a message has appeared saying "Insufficient broadband speed detected".
I have run speed tests that show I am getting close to the promised 40Mb speed. I get the same results running the test immediately after the "insufficient broadband" message. I am running the players on a modern laptop connected to the Plusnet router using ethernet over powerline adapters.
Is there anything I can do to improve the streaming TV, or is the poor quality something that I should have expected?

[#2]

Are you watching iplayer on your TV, a laptop, or something else?
I have a 2.5Mb ADSL connection on PN and I can watch iPlayer on my TV and laptops without buffering, albeit in the SD picture quality setting, so on iPlayer the HD icon is set to Off.
If your TV and laptop are connecting wirelessley to your router, you might want to install inssider on a laptop and use it to see if you are competing with many other wireless users near to you.
Who was your previous broadband provider and did you have the same problems with them?

[#3]

I either watch the streaming TV on a laptop or on a dumb TV connected to the laptop via HDMI. The laptop is connected to the router using powerline adapters. The laptop wireless is turned off.
[Incidentally, I the router's default channel was close to a neighbour's channel so I manually changed it. That is the only router change I've made.]
My previous broadband provider was TalkTalk. I did get buffering problems with them but I only had an ADSL connection. Hence my move to fibre with Plusnet.

[#4]

I suggest you try the laptop plugged directly into the router as a test.  This will rule out the powerline adapters.

[#5]

I had issue streaming over power line adaptors, so I'd second the advice to eliminate those and try again, and if they are the problem, replace them with a hardwired network cable...

[#6]

I was going to suggest the iPlayer diagnostic speed test, however they seem to have removed that earlier in the year. Therefore I'd echo the remarks of the other posters in this thread, eliminate things one at a time.