HI Nobby,
Currently on my 3gs i have the following:-
QCT Viewer:- See my review last week, nice but you obviously need to have memory map maps, basic allows you to view the map but no search facility etc, v1.2 will be out shortly with some good improvements. No OS Grid as yet. Really ownly of use if you already own memory map.
RouteBuddy Atlas:- cheapest mapping out their, but does not give you the ability to navigate only mark positions and track. Map scrolling is good and smooth, had some issues with uploading the maps to teh iphone, but at £99 of UK 1-50000 very good price. Lots more developments on way, excellent support.
GPS2OS - gives OS Grid Refrence, my phone is jailbroken which is simple and not illegal, and i run this at the same time as either QCT Viewer or Atlas. Multiflo available via cydia when jailbroken will allow this, i can see where i am on eh map and this give me the OS refrence.
CopPilot UK, allows you to navigate to an OS grid refrence NOT!!! it has this function but it does not work, fingers crossed soon. UK Roads only No OS info as you require.
MotionX GPS, excellent navigational aid, you can us it in map mode and just navigation. My favourite feature even though it does not allow OS grid system is that if you want ti walk on a bearing of say 200 deg for 1 mile you can put this in and it will project a waypoint, you then follow it to that point, so like walking on a bearing but it doesn't matter if you drift as the original point is creatd a s waypoint and it will selft correct, very handy in a whiteout!. Good tracking and import export of waypoints.
Tomtom - shocking, jumps about all over the placem copilot is much better as a sat nav.