Tried for hours to get this to work and ready to throw laptop out of window.

What I'm trying to do :

enter mileage into A1

For first 200 miles charge £0.40 per mile for miles above 200 Charge £0.25
per mile and enter combinded charge into A2.

and if mileage is less than 200 in A1 carry balance (200-A1) over to B1 to
increase B1 £0.40 rate i.e 200 plus balance from A1

Hope that makes sense . Many Thanks for any help.