I used the Dell support tool to create the image on the usb drive. Since that was done connected via a hub, I tried redoing it connected directly to the other pc. It created the ISO image much quicker, but still no option to boot from usb. I had already tried the legacy non-uefi boot option in the BIOS but it made no difference. Somehow I did find a way to select the usb (though it was labelled as "dvd" with the usb's name!) but still the "install" option reported there was no bootable source. So I tried again and this time I tried using the "custom install" option and it started to install. It stuck on "getting ready" for about an hour, and then went through all the other ridiculous Microsoft delays (please wait, almost there etc) but eventually reached the account set up stage. So far it seems to be working fine.....
Won't boot so I created a bootable usb following the instructions. The instructions are incorrect though and this option doesn't show up: At the Dell logo screen, tap the F12 key until you see the Preparing one time boot menu in the top-right corner of the screen.  
Inspiron 24-3455 won't boot image on usb drive