GeoUK and GeoUK.OSTN
This is a simple to use Geodetic Library for .Net that will allow simple transformation to and from British National Grid. GeoUK.OSTN http://www.nuget.org/packages/GeoUK.OSTN/ is an extension to GeoUK that provides OSTN02 transfiormations, see the NuGet project page (http://www.nuget.org/packages/GeoUK/) and the associated CodeProject article (https://www.codeproject.com/Articles/1007147/Converting-Latitude-and-Longitude-to-British-Natio) for details on how to use these packages.
Source code is available here https://bitbucket.org/johnnewcombe/geouk/src