I've got a 1992 1.8 litre Driver with 230k miles on the clock!

I mostly go to work and back which is a 24 mile round trip along an A
road so I usually go at a comfortable speed 50/60. The last few times
I checked Ive got 29mpg.

However in the past I've recorded 36 mpg and also worse, 26mpg.

No work has been done on the car except for a basic service and mot.
Apart from adjusting the mixture and idle screws once or twice to get
the idle speed correct.

Is there anything I can check to see if its running as it should?

Also if I put my foot down in 3rd or above at lower revs the clutch
slips, its been doing this for ages, but for normal driving you dont
notice it. How long should I expect it to last before it gets worse?