Hi David
I was having precisely the same problem with a London Clock in the autumn. Now it doesn't run four hours slow it just roams and roams like it can't find the time signal.
I am going to try some of the suggestions from the National Physics Laboratory who are responsible for the time signal now Greenwich has shut:
http://www.npl.co.uk/server.php?show=ConWebDoc.1002
Watch this space!
Drizzle
December 2008