Sorted it, seems like the SPNs registered directly against my servers hosts names, and not the service account I set up during the custom installation path.

setspn -D MSSQLSvc/IA-I-C3-SQL-01.ad.vpc7.infra.domain.com IA-I-C3-SQL-01

setspn -D...

PS C:\Users\james.shaw\Desktop> .\sql-spn.ps1

cmdlet sql-spn.ps1 at command pipeline positcomn 1
Supply values for the following parameters:
TargetSQLInstance: ia-i-c3-sql-02
--> ia-i-c3-sql-02 <--
ad.vpc7.infra.domain.com


ServerName          ...