On my trips I often get reading of 68/70 mpg, but last week happened to look at screen and it was reading 70 plus mpg later it went to 80 mpg , so its gone wrong  but how do i do it?

?

How many miles did it keep reading that, and were you rolling along, maybe off the accelerator and getting great efficiency?

 

Do you actually ever get maybe 750-800 miles out of 45 litres of go go juice?

The trip information in memory 2 will give you long term mpg over its recorded miles as long as data has not reached maximum and rolled over to zero.

5 hours ago, pastiche said:

I often get reading of 68/70 mpg, but last week happened to look at screen and it was reading 70 plus mpg later it went to 80 mpg

Yes and? I've seen a sustained reading of about 120mpg with the car in "instantaneous consumption" mode and running downhill.

2 minutes ago, KenONeill said:

the car in "instantaneous consumption" mode

No doubt the OP had unwittingly switched back to that mode after looking at another parameter.