WOL BIOS settings for Dell OptiPlex 3020

We have no issues with Wake on LAN (WOL) with our Dell OptiPlex 3010 machines, but I cannot get our Dell OptiPlex 3020 machines to boot in the same way.

In the BIOS I have enabled WOL and PXE booting as follows:

System Configuration > Integrated NIC
Enabled w/PXE is ticked

Power Management > Wake on LAN:
LAN with PXE boot is ticked.

If I send a WOL signal, without PXE, I simply want to boot the machines normally into Windows, the 3010 machines will wake and look for a PXE server, when none is available, they will exit boot from LAN and boot from disk. Which is how I want it to work.

But the 3020 machines - which are set up as close to identically as the 3010s as possible - will also wake and look for a PXE server, but when none is available they will ask for a key to be pressed before they will boot from disk.  That's no good; I want them to boot into Windows (without user intervention) if there's no PXE server available!

If I set the 3020s to simply "Boot from LAN", then sending a WOL signal will correctly boot them into Windows. But that's no good either, as I want to have the option to PXE boot the machines remotely from time to time, as I want to be able to re-format a whole room of PCs using SCCM occasionally.

Does anyone know what the correct BIOS settings should be, to get the same result on the 3020s that I get on my 3010s?

Thanks in advance.