SQL Job PowerShell task fails same command succeeds as user
cd C:




Get-ChildItem -Path \\FileServer\SQLBackups\SQLServer\ -File -Recurse -Force | Where-Object {($_.LastWriteTime -lt (Get-Date).AddDays(-20))}|Remove-Item


https://www.sqlservercentral.com/forums/topic/powershell-for-network-path
http://www.midnightdba.com/Jen/2013/05/quick-tip-navigating-to-a-unc-within-sqlps/

Adding the cd C: line to my Powershell task in SQL Job allowed UNC references to resolve remote server file directory.

To quote a wise man:
"SQL Server Agent integrates with powershell in very unintuitive ways sometimes"

Thanks for your help, Peter.