Garmin Etrex - ORdnance Survey Grid reference nos

You need to make sure your GPS unit and Google maps are using the same map datum.
In your user manual:
http://static.garmincdn.com/pumac/eTrexLegend_OwnersManual.pdf
Page 44, Setup Options, Units, Position Format: I suggest setting to:
hddd.ddddd as this is the digital format and easier to read

In Google Maps, right click - Drop Latitude/Longitude marker and this should show the co-ordinates in digital degrees. Alternatively, you can hit the SEND button in GoogleMaps, and it will send the marked location to the GPS via the Garmin website. You may have to install the Garmin plug-in into your browser first, but it's well worth it.

If this solves your problem, please come back and rate our solution, thank you.

Unfortunately Garmin satnavs do not allow OS Grid References instead of Longitude and Latitude. However if you need to find the Longitude and Latitude of a point it can be done by going to http://www.gorissen.info/Pierre/maps/googleMapLocationv3.php Zoom in on the location you want and left click on it. The longitude and latitude will appear in a box to the right of the map. You need to set the co-ordinate system of the satnav to match the longitude and latitude format of the satnav. To do this touch Where to, Co-Ordinates, Format. Enter the co-ordinates and touch next. It's not an ideal solution but it might help you out if you're stuck.

If you mean the WGS or MGRS grid reference, you have to change it in the 'navigation' options.

Setup > Position Format > Page 44 of your manual.

You'll have to do this online
http://www.cellspark.com/UTM.html

BNG stands for 'British National Grid'

1. Go to the Setup menu
2. Select Units
3. Change the Position Format to hddd°mm.mmm'

If you press in the control stick, it will make a waypoint at your location and you can see the co-ordinates. The numbers can be displayed in various ways, all depends the settings you have used in the Etrex.
Your bearing will be on your Navigation Page.
http://www8.garmin.com/manuals/eTrex_OwnersManual.pdf

You should be able to get it from the set-up menu - units - british grid.

BNG stands for 'British National Grid'

1. Go to the Setup menu
2. Select Units
3. Change the Position Format to hddd°mm.mmm'

UTM GPS position format, WGS 84 datum point if my memory serve correctly