Its probably obvious but how do I delete obsolete user arps?

Press [UTILITY]
Touch “Contents” > “Data Utility” > find the Folder labeled “Arp”

You can open the folder to review the contents. This Folder contains your User Arp data and allows you to manage it.
As many as 256 User Arps can be stored in the folder.
You can tap the one you want
A pop-in menu appears to “Rename” or “Delete“ the item.

If you have several that you need to Delete, you can activate the JOB option (green) upper right corner.
This will activate the option to Select All, or you can place check marks next to all the ones you wish to Delete.

Hope that helps.

Thanks for your help. That was exactly what I needed to know.