OK, screws are T5 - driver ordered.
Updated to 595 using the 4.60 firmware yesterday, and now have maps working happily despite the lack of NVRAM (Same region can be split between the device and a μSD card, luckily). Some jiggery-pokery involved in getting maps to work, as it has none and is not an LM device, and some more will be involved going forwards, but I can live with that.
Shame that the 595 upgrade means you lose the HD logo on boot up, and surprising that having lost that, it's not replaced by the Garmin one from the standard Zumo. Kind of hard to get my head round that.... Still, it's the only minor annoyance. (So far at least!)
Jim