I have found a solution!
It’s all about the output function rather than the save function.
using get_field('field_name') wasn’t outputting a value
I needed to use get_field(field_id) (it will look something like this – field_52d84a083f9c6)
I generally do use field ID’s as they’re supposed to be more reliable, for some reason I used the field name here and that was causing the problem.
Hope that helps someone
