My old R50 Cooper returned about 400 miles to a tank and my R53 Cooper S (with a reputation for being thirsty) usually returns 340 miles to a tank and, taking it really easy, I can see 400 miles out a tank on the
MCS. So, you've definitely got a problem there.
Putting the air filter completely back to standard with a new filter would be a very good start and then check the economy again. I wouldn't bother with the spark plugs, other than taking them out and checking the colour, they should be a light grey colour.
If the economy is still bad, it's time for a trip to the garage...
Tigger.