Hi Gerrit,
Following has been a very useful site for me which is a lovely little reference for geo distance calculations (I implement some of the examples in Javascript with success).
Anyhow, plugging John O'Groats (top of UK) and Land's End (bottom left UK) gives 969.3km using the great circle distance ellipsoid formula:
970Km I believe is the generally recognised distance between these two points. Just wondering why 996Km with SRID 4326.