I emailed Astra in Luxembourg who run the Astra 2D satellite that the BBC and ITV use.  Lots of people have recently been having problems with mainly ITV but also some BBC regions.  I enquired if they had reduced power on the satellite. Today I received the following response:

Thank you for contacting ASTRA.

There has been no change to the ASTRA 2D transmissions. Possible cause could be an incremental level of adjacent satellite interference, because of additional transponders being operated at adjacent orbital positions. Repointing the concerned antenna's could be a solution.

Kind regards,

The ASTRA Team

So, no definitive reason or solution.  The only solution I know works (for people with dishes of 1m or less) is to upgrade to a 1.2m dish.  This seems to cure all problems.

Nigel