Massive fibre speed slowdown on ptn-ag01 tonight 8 May

[#1]

My download speed hit the wall about 30 minutes ago, using ptn-ag01. Down from the low 40s to the low teens. I've done a soft reboot & the speeds have recovered, now on pcl-ag02.
Why does this happen? In simple words please, I'm no compuer genius. 

[#2]

Not aware of anything amiss, but whatever it was could be anywhere between you and the www. Have you had any thunder and lightning near you this evening?

[#3]

I had a problem yesterday but it picked up last night, then this morning was as bad as ever.
My speed test was between .70Mb/s and my normal 4.9Mb/s. Unfortunately I switched the router off accidentally before I checked what gateway I was on.
All back to normal now. Fingers crossed it stays that way!

[#4]

We had no reported issues, if it happens again could you run a speed test and run further diagnostics.
@Mayfly
You were connected on pcl-ag04 during this time.

[#5]

Thanks Chris, I most certainly will.