Nav calculations
For the PPL navigation exam you are provided with 1:500,000 southern chart. Use of a standard navigation scale ruler and a protractor is all that is required. On such a chart scale may be considered the same in all directions. However to answer your question - 1 minute of arc on a great circle = 1 nm but only on a great circle. You are not allowed a calculator but must use the whiz-wheel for all calculations (or long hand if your good at sums but to get the same answer as the CAA use the whiz-wheel for that is what they have used).
You will not be required to scale using change lat/long. You will be required to identify a position by reference to lat and long figures. Seconds may be given as 60ths of a minute or in divisions of a hundred. Count the digits given to be sure.