From Euston additional departures
These services to operate every night - the "insiders guide" says from 18 July to 14 September
They seem to be showing up on Open TrainTimes... e.g. http://www.opentraintimes.com/location/EUS?day=31&month=07&time=0100&year=2012
Oh look, they're in this timetable too...
Isn't it well known there are extra trains on the ECML, WCML and GWML for the Olympics? General consensus I saw on another forum a while ago was it was to be 67s on the ECML and GWML, with 90s up to Manchester on the WCML.
No doubt Leeds and/or Newcastle were the ECML destinations, Cardiff was the only one I recall for FGW. 0100 or something daft like that I believe.