mad_jock, your comment about 'Mode S' raises an interesting question: Instead of a VFR flight plan (given the uncertainties reflected in the various responses in this thread -- especially for the UK), wouldn't the digital track history (from Mode S tracking by a radar service) be the best way of estimating the position of a lost aircraft which disappeared from the radar ? Does anyone know if and for how long these digital records are retained for ready retrieval in an emergency ? For example, are they kept for minutes (not very helpful) or hours/days/weeks/months ?