Why not just use the Ordnance Survey website?
https://osmaps.ordnancesurvey.co.uk/osmaps/

If you enter a Lat/ long reference into the search box in the format 56.8762572609,-5.3215554206 it will centre the map on that point and you then click on the "Grid Ref" tool and a pin is dropped showing the full OS Grid ref.

Conversely, with the grid ref tool enabled, you can centre the cross hairs on a map feature, or enter an OS grid reference into the search box and the lat/long coordinates will appear in the URL at the top of the page. The two digit number after the comma is the zoom level of the map on display.

If you are a registered user, planning and storing a route on the OS website is really easy and gives you a printout of waypoints and elevation changes.

That was fast, thank you very much, I have it all sorted now!

Andre Hilhorst
Haarlem
The Netherlands