There are a number of possible reasons for the difference between the pixilation on your TVs and methods that could improve your signal so that it is good enough in the evening.
(1) Some boosters that have multiple outputs favour one of the outputs over the others.
The second possibility is that Humax tuners tend to cope with poor signals better than Dion receivers. The different Sony models vary in their ability to cope with weak signals. To see if this is the case try swapping the cables over to different outputs
(2) Some of the leads from the booster may not be as good quality as the others.
(3) I hope that your description of having a 'digital aerial' is your description and not that of whoever did the install for you. Digital broadcasts are broadcast using analogue radio waves and what is a good tv aerial design before digital switch over remains a good design afterwards.
Sometimes wideband aerials are described as 'digital aerials'. These are popular with some aerial fitters but are frequently not suitable for those in weak signal areas. Have a look at the following link for what are suitable for your transmitter.
http://www.aerialsandtv.com/digitalnationwide.html
(4) Apart from checking the suitability of your aerial you could also upgrade the down lead that comes off the aerial to one of satellite grade.
http://www.aerialsandtv.com/cableandleads.html#H106CT100
(5) Should you already have a suitable roof aerial and a good quality down lead then installing a masthead amplifier would also improve your reception.
NS
August 2012