pedro2000uk

Sounds like excellent progress lloowen!

Could you upload a photo that shows the whole dish face and those arms?

Sounds like you should start again with those support arms... before you do that you must establish it's best position focal length wise and work from there.

Where the focal point is exactly on an LNBf is one of life's mysteries... it's about 40mm in from the face and then allow for fine adjustment.

To aim the LNB holder to the middle of the dish is easy.. just look down it and sight it in line... a small length of 40mm plastic waste tube in the holder is helpful..

The main thing is the F/D ratio once you have the focal length OK and remade those arms ... with an LNBf (most with an f/d ratio of .6) on a dish with a 50cm focal length that is about the same as some 60-70cm dishes and that's all it might see.

If you don't mind trashing a cheapo LNBf (5 euro etc..) as an experiment (before buying an adjustable feed horn and C120 LNB)... you could try sawing the feed horn right off one... that plate that holds the LNB will help act as a bit of a scala ring and you can make a better one later...(and seal the lnb sawn off front)

EDIT ... I would do all this on an easier satellite or beam first.