Mercedes bourton sat nav

Is it inaccurate all of the time or only intermittently? If the latter it may be that something else in the van is interfering with GPS reception.

If it's consistently inaccurate, one possibility is that you need to change the map datum. It sounds unlikely, but you do get this effect if the geodetic datum is intended for the UK - they can get quite inaccurate if you go outside of their normal range of accuracy. I think WGS84 is the most commonly used datum, but there are others - is there a setting for this in the setup options?

I don't understand the details in full (or anywhere near), but as I recall, the geodetic datum is what is used to convert a grid based map reference so that it matches the weird shape of the earth's surface (it being an oblate spheroid rather than a simple sphere).

[To those who care - yes, there was some Google Thinking involved in the above reply!]

I'm really only stating my own personal experience when setting up a Garmin hand-held GPS receiver - it was configured to use a different map datum (because it was sold for the US market) and so was a significant distance out in the UK - I solved the problem by selecting OSGB instead of what it was supplied set to.