I have a small chalet park in Scotland and am about to replace curtains. The chalets are single story and sleep between 4-6 people in 2-3 bedrooms.

Do the curtains need to be FR treated?

Depends on your fire risk assessment, they are a flammability source which could be a surface spread of flame risk but would they be easily ignited. There is no clear guidance to say they should be flame retardant it is up to the fire risk assessment to decide if they need to be FR.

Check out  http://www.gov.scot/Resource/0041/00418066.pdf  and  http://www.gov.scot/Topics/Justice/public-safety/Fire-Rescue/FireLaw/FireLaw/SectorSpecificGuidance/smallsleepingaccomodation the best guidance I could find.