If the Astra and Eutelsat satellites at 28 degrees East had all ceased transmitting simultaneously, I think that there might be more than just one question about it here!
The transmitting power of an Astra satellite is about 40 watts (i.e. roughly the same power as a tungsten light bulb in a desk lamp). That power has to be spread across the whole of the UK (and much of western Europe) from a distance of around 23000 miles away. So it's unsurprising that the signal strength at your dish is quite low to start with. Atmospheric conditions, both in terms of pressure and precipitation, can add to reception problems. (Snow storms certainly don't help!) Interference from nearby electronic devices can also make it harder for your satellite receiver to 'lock onto' signals.
Your first step should be to check that your satellite dish (and it's LNB) are in
exactly the right position. It could be that, because of a slight misalignment, you're getting a lower signal strength than the maximum possible (which is good enough to work in good reception conditions but not good enough at other times). You can do it yourself
http://www.maplin.co.uk/satellite-finder-48325
or call in a professional.
Then consider the possibility of interference, both from the cable and via the mains. All electronic devices (such as, for example, your next door neighbour's TV set or your own inkjet printer) create electronic interference, often even when they're turned off (unless they're disconnected from the mains). You should try to ensure that both the cabling and the set-top box are as far away as possible from possible sources of interference.
Interference can also be carried via the mains supply. Mains conditioners aren't cheap but they're often necessary!
http://www.play.com/Electronics/Electronics/4-/3518993/511014852/6-Way-Surge-Protected-HIFI-Home-Cinema-Mains-Extension/ListingDetails.html?_%24ja=tsid:11518|cat:3518993|prd:3518993
Chris