SHJ, pleased you have nailed the problem. The signal quality can be affected by several things.
Firstly, was the used LNB you were given a good one? New ones are not very expensive.
Secondly, as mentioned above, 'skew' is another area to look at. Simply put satellite TV signals are polarised either in the vertical plane or the horizontal plane. The LNB must be aligned i.e. 'skewed' to match the angle of the polarisation.
Use
this link which will tell you the azimuth, elevation and skew angles for your exact location (select the Astra Satellites at 28.2°E). The skew is normally only a few degrees and from the link the direction of rotation is viewed from behind the dish and minus is anti-clockwise. Some LNBs have degree markings on the body but if not think of a clock face and each minute mark represents 6° to help you visualise the amount of skew needed.
If your signal quality is low and are getting a picture it is because there is sufficient signal to get a picture. Bear in mind digital signals give a picture or not they will not give you a fuzzy poor picture but if the signal is too weak your picture will freeze or start becoming 'blocky'.
Given your previous comments about losing the picture during rain I suspect, LNB, cable and dish being satisfactory, that you have an alignment issue. You are in the UK I presume?