. Is it possible that where you get your fuel has been selling dodgy
Old fuel which has lost some of its octane ?
Fuel from various places, John.
Since you just replaced the spark plugs, maybe check that they are sparking properly?
The increased fuel consumption came before the new plugs, and was one of the reasons for changing them.
But checking spark might still be a good idea.
Speaking of spark plugs...Did you buy them from a reputable seller?
There are a lot of fakes out there... 
https://youtu.be/vpH4O14EBxA?si=AumCg0AnDOvdbIUO
I don't know how reputable they are, but I bought NGKs from a bricks-&- mortar source in Sydney, so they should be good.
Place in Castle Hill called the Automotive Superstore.