It is often thought that Old English /r/
was a "continental"-style trill. Actually, West Saxon /r/ seems to have been
retroflex. Tristram (1995) supposes that the retroflection was a
spontaneous innovation that developed in Wessex about the 9th century and spread
throughout southern and western England as far as Watling Street. Tristram notes
that the West Country retroflex /r/ has also crossed the Channel and is now
commonly heard in Trégor Breton (one observer has compared that dialect
to "Welsh as spoken by Americans").
The Old Anglian quality is harder to
determine -- a genuine trill, perhaps, or a weaker rhotic like the modern
RP alveolar fricative(/approximant), or a mosaic involving both realisations. In
either event it painlessly merged with the apical-trill pronunciation of the
Danish settlers. Whatever the immediate outcome of the merger, it was
eventually lenited and de-trilled, and by the 18th century people were
dropping their /r/'s in syllable codas. This weak alveolar
sound became a feature of the standard British accent via
the prestigious pronunciation of the East Midlands and London, and has since
encroached somewhat on the historically retroflex areas.
Piotr