Hmmm...

> When I booted from Disk 1 I could access DU from the menu drop down, but it offered only Erase & Install.

That doesn't sound correct; erase and install is an installation option, not an erasure option. I'm not certain what you were looking at.

> I back tracked and allowed the installation to run, filled in the required details and accessed DU via > Apps > Utilities >DU, but the erase option was greyed out.

That's because you can't run "Erase" on the volume into which you're booted.

> I then rebooted from Disk 1 but instead of first choosing “English,’ I went straight to the drop down menu > DU > name of HD > and got the erase options.

Isn't that how you got to run "Erase" the first time?

> The 8 way random write is running as I write this.

Sleep tight, Kate! smile

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire