Google &c also tend to use the WGS84 geoid 

"WGS84 was designed for navigation applications, where the required accuracy is one metre or lower. A high-accuracy version of WGS84 known as ITRS (International Terrestrial Reference System) has been created in a number of versions since 1989, and this is suitable for international high-accuracy applications (it is used mostly by geoscientists). However, there is a problem with trying to use a global coordinate system for land surveying in a particular country or region. The problem is that the continents are constantly in motion with respect to each other, at rates of up to 12 centimetres per year. There are in reality no fixed points on Earth. In common with the rest of Europe, Great Britain is in motion with respect to the WGS84 coordinate system at a rate of about 2.5 centimetres per year. Over a decade, the WGS84 coordinates of any survey station in Britain change by a quarter of a metre due to this effect, which is unacceptable for precise survey purposes.
For this reason, the European Terrestrial Reference System 1989 (ETRS89) is used as the standard precise GPS coordinate system throughout Europe. ETRS89 is based on ITRS (the precise version of WGS84), except that it is tied to the European continent, and hence it is steadily moving away from the WGS84 coordinate system. In 2000, the difference between the ITRS (precise WGS84) coordinates of a point and the ETRS89 coordinates is about 25cm, and increasing by about 2.5 cm per year. The relationship between ITRS and ETRS89 is precisely defined at any point in time by a simple transformation published by the International Earth Rotation Service.
The ETRS89 coordinate reference system is used as a standard for precise GPS surveying throughout Europe. Using ETRS89 you can ignore the effects of continental motion: to a high degree of accuracy, the ETRS89 coordinates of a survey station stay fixed, as long as there is no local movement of the survey station. ETRS89 has been officially adopted as a standard coordinate system for precise GPS surveying by most national mapping agencies in Europe, including Ordnance Survey." [http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/coordinatesystems.html]


The differences may be vey minor, but are greater than the somewhat spurious precision reported by Google maps for example.
Less up to date converters than the one Mike suggests may be a lot worse.

Nick

>The site http://gps.ordnancesurvey.co.uk/etrs89geo_natgrid.asp is useful for
>going from type of co-ordinates to another.  Google does tend to use a
>decimal Lat and long rather than minutes and seconds.
>Mike Clinch