Bit of an update: I've managed to get the USB to be detected now, I formatted the USB pen using Rufus as an MBR partition for UEFI but was still having no luck with it being detected. I read elsewhere that the bios is 32-bit only, so the 64bit efi files used to boot weren't being recognized. I threw the 32bit boot files on the pen and was detected straight away!
However, Windows setup was reporting that the CPU wasn't 64bit and can't be installed (even though in Windows it reports it as 64..).
I'm about to try a 32bit install and see how I get on.