Replace your cable with a fully-shielded type such as WF100.
Replace your masthead amplifier with a REALLY low noise type. The best I've found so far is 1.4dB with variable gain control.
http://www.satcure.co.uk/accs/page11c.htm#variable
You can install it in the loft.
The best aerial for Crystal palace ought to be a Blake DMX10A (group A) aerial. I think CPC sell these. The Blake DMX10A has 12.8 dBd (ie 15.0 dBi) of gain.
The only way to do better is to get it higher or install a DAT45 or a chimney-busting DAT75. However, this is 7 feet long!
Obviously another possibility is to buy a used Sky Digibox, a quad-output LNB and another £20 Freesat card.
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