The MSF transmitter at Anthorn is from time to time off air for maintenance and at those times the MSF clocks have to run on their own internal Xtal which may not be as good as it might.
Also its quite easy to have the 60 kHz signal interfered with or blocked completely by steel framed buildings and the like especially in the south of the UK seeing that Althorn where it is now is in a remote bit of Cumbria...
Its run by the NPL and all you need to know is on their website...