After having closed the payroll, I realised this month that I had marked the wrong employee as a leaver. This can be easily done if you are toggling between different modules in Kashflow and do not check you are inputting the information in the correct employee above.
However, when I restored the payroll to correct it, I was unable to remove the leaver date from the employee. The only option I had was to ADD THE EMPLOYEE AGAIN even though they had never actually left. Now the payslip I send to him says that he started this month (when he actually started in 2016!).
Apart from all the work needed to create the employee again, add opening balances, add him back to the pension plan, etc., it also has caused unnecessary confusion with my employee. There should be a simpler solution where the leaver date can simply be removed.