BR500 IPSec - Can't set Aggressive Mode

I am replacing an old FVS124G with a BR500, but I'm having extreme difficulty in manually configuring an IPSec VPN. I need to use Aggressive Mode as the other end point of the tunnel has a dynamic IP address so I have to specify its FQDN.

I am using IKEv1 (which permits Aggressive Mode), but when I select it and click apply, the policy reverts to Main Mode. In theory this should work, but it dosen't.. and I've not been able to turn up any answers online. Hence my post here.

I'm running Firmware v5.8.0.1 and I even tried downgrading to the previous version to see if that would help.. but no joy.

I'm very much hoping that someone can throw me a bone and help me resolve the issue.

Thanks in anticipation,

Anton