Let me continue the griping...
I started off the morning before breakfast with a quick email check and got bombarded with advertisements

That could only mean one thing, that the Pi-Hole has failed. If it goes offline, then there is no DNS to successfully connect to the internet. So, it had to be something more insidious than just going offline.
Going to
http://192.168.0.10/admin gave me an address not found error. Looks like Pi-Hole is offline?

ping 192.168.0.10 gave me sub millisecond response. Okay, so it is still online?

I spent the day checking the connection of the Pi-Hole, the router, the switch and my usual computer.
Nothing made sense?
Do I have a virus on my computer? Is there a virus on the modem, router, file server or switch?
Everything got updated and scanned, especially now that DOH is being introduced on some things. Nope; that did not help.
Physically unplugging the Pi-Hole did not change anything; something was still answering pings and http to that address would not work.
Out of desperation after supper I started pulling ethernet cables.
That gave me results almost immediately.
My computer is connected directly to the router, due to close proximity. With a non Pi-Hole DNS entry in the router, my computer started working again.

ping 192.168.0.10 is now failing with the Pi-Hole disconnected. Finally, something makes sense!
If only I had thought earlier that something else could be spoofing/colliding with the Pi-Hole on 192.168.0.10!
In just a few minutes I disconnected and reconnected various cables and found the culprit.
SWMBO's ultra secured work computer on 192.168.0.153 was responding to requests meant to go to 192.168.0.10.
I can only assume her computer was pushing all DNS requests through its VPN to her office.

SWMBO is now banned from putting her work computer on the home network.
At least until Tuesday....

Can I bill her work for 11 hours of overtime during a national holiday? Of course not

Finally, Pi-Hole is back up and running and no more ads.
It is important to filter out those ads. 50% less data on a slow connection is significant!
Maybe tomorrow will be the day to check the internal HP DMM battery...