No.
Next, nurse.
Next, nurse.
Does the Montana come with turn by turn road navigation? (looking namely at the 600 at the moment... don't need camera so see no point spending more for it)
I get the feeling it doesn't? so what do I need to use turn by turn navigation (Road) but also be able to access OS Maps... just interested in UK at the moment
do I need something like GB Discoverer?
What does it do straight out of the box?
Try a phone with Osmand, Sygic or NavFree. I have been impressed with all three. You can load routes you make on a computer as gpx files and they all offer turn by turn directions. They work on Android or iPhones and are either free or very cheap. You can certainly display OS maps on them although you will have to pay for an OS map. I don't think you can get turn by turn directions for an OS map but it might just be possible to display one as an overlay whilst getting audio prompts from Osmand. A waterproof or water resistant housing for the phone needn't be very expensive. I use one of those zip type things and they are OK in a reasonably heavy shower.
Have a Garmin Zumo 220 for sale £155 posted complete with bike mount, car cig lighter type cable and a wire in cable. with 2014 maps.