I fired up the computer in the same room as the router is.
Wireless result was
http://www.dslreports.com/speedtest/31730854

Ethernet result was
http://www.dslreports.com/speedtest/31731938

tbb:
Wed 04/04/2018 18:00
37.64 Mbps
35.58 Mbps
7.17 Mbps

So there doesn't appear to be a problem in there, wireless or ethernet.

So at a bit of a loss. There hasn't been anything new introduced to the rest of the house that I can think would cause the problem either.

Thanks for your help.