Pixelation on certain sky channels?

It could be the weather, or could be that other channels have been added to the multiplex and used up most of the available bandwidth. When digital channels (satellite or terrestrial, like freeview) are broadcast they are split into groups, called multiplexes. Each of these multiplexes are allocated a certain amount of space to squash all of the picture information into. Due to the way that the picture are transmitted digitally they sometimes require more or less of this space. for example a fast moving/changing picture will need more digital space than a still, very plain picture. If there is not enough space available then the picture will look 'blocky' or pixelated for a short while until there is enough space to transmit all of the information correctly