So I solved this problem in the end, but not a satisfactory one and wondering if one of the fine people on here can enlighten me.
B+ with Jessie, I couldn't get a Netgear WNDA3200 2.4+5Ghz dongle to work-when I say work, it came up with lsusb as "no Firmware". If you Google, you'll see this a lot so I made sure the firmware-atheros and usb_modeswitch were installed. Same combo worked on a Pi 2, so I was scratching my head. Eventually plugged a USB 2.0 Belkin 4-port hub into the B+ (as hot-plugging on a B+ for me results in a reboot every time!) and hey presto, the Netgear dongle came up every time when connected to the hub ( rebooting the Pi B+ each time)
I dont know why this is, I take it it must be something electrical that the dongle doesn't get "initialised" on its own?
Not a big deal but would like to know why this would be the case?