Is anyone able to enlighten me as to why the clocks on the Docklands
Light Railway departure boards at all the stations I saw today were
about 6 minutes fast? Are they using a GPS NTP server? Too much snow
on the antenna? How are clocks synchronised on the real railway?

The online versions of the DLR PID's are here (the time appears to be
closer to the truth online):

http://www.dlrdaisy.co.uk/daisy/pop/pop05pid.htm
http://www.dlrdaisy.co.uk/daisy/caw/caw05pid.htm

substitute "pop" or "caw" (Poplar/Canary Wharf) for the station of
your choice and 05 for the day of the month.
or click on the map he http://goo.gl/w1oqm for today's live boards.