I'd guess NHS have been desperate to settle this, and Sandie has refused unless NHS concede they were wrong which NHS unwilling to admit. Or NHS wanted a confidentiality clause which she's refused to agree to
Settlement offers are usually made without admission of fault, and often with requirement of confidentiality about precise terms
If her tribunal application seeks X, Y and Z, and NHS offer that, she'd be found liable in subsequent costs as tribunal would be unnecessary but otherwise No she's not obliged to accept any offer