SQL Server 2017 Installation Errors
Looks like we have located the issue, DNS entry for the domain is wrong which is being pushed out via group policy.  

Thanks to Pinal Dave for his blog post

https://blog.sqlauthority.com/2018/07/21/sql-server-install-error-validation-for-setting-agtsvcaccount-failed-error-message-the-rpc-server-is-unavailable/

Just in case the link becomes stale:

Against the properties of the Ethernet adapter, IPV4, DNS tab, in the section for 'Append these DNS Suffixes' we have an incorrect entry for domain, this is controlled by group policy (for us) we are resolving this now.