Returned from holiday to find broadband speed slowed badly and erratic

[#1]

Just got back from hols and my line speed has fallen down badly, getting this result

I'm using a MAC and nothing else is running when the test above took place. I have an Asus RT-N66U router which is normally rock solid.
Any ideas what is going on? I should be seeing at least 50Mbps I think.

[#2]

Are you connected via Wireless or Ethernet?
If Wireless, what speeds do you get connected via Ethernet?

[#3]

This was over ethernet last night. Tonight over WiFi from my MAC laptop I get.

Intermittent issue of some sort?

[#4]

Possibly, does this happened regularly at the same time each night? Are there any patterns to it that you can see?

[#5]

Not sure that there is any pattern to this. I set up a ping monitor with f8lure which gave the attached results over the past 24 hours. There are clearly some response spikes but no pattern.