Choose Wifi Network Option Missing after Holding down alt to Boot

Hi Guys

I switch on a MacBook while holding down the Alt key. This brings up the HDDs available and normally

also has a Dropdown which says choose Network which allows you to select a WiFi network

(I use this option so my Deploy Studio/Netboot Images are loaded as well.)

However recently when I hold down Alt I see the drives but no option to select wifi network.

This Macbook (White Macbook) has had 10.11 installed on it previously

Anyone else noticed this and if so how did you get around it?