Tax on future payments on exercising options on unapproved share scheme

I am exercising share options from a private employer on an unapproved share scheme. The payment terms are that the total value is paid out over 5 years, 20% each year. Interest is due on the outstanding balance but the debt is unsecured. Is PAYE and NI due on the full value in the year of exercising the options? Is any allowance made for only receiving the value over 5 years. What happens if for any reason, I don't receive the future funds?

Hi,
Please have a look at the guidance at:
HS305 
Employment-related shares and securities (Self Assessment helpsheet HS305
Thank you.