I've been communicating with Dr Sam Morrell who was kind enough to look at some of my data.

He thinks I've been successful to capture a signal at 1420.4 MHz.

I've now ordered the horn to be made in Aluminium at a local Engineering company.
Also I've tried to make the Wave Guide out of Aluminium sheet.

Problem is that the Aluminium is too thick at 2 mm. I can just about get about 75 deg but just cannot bend it any further.

I now have some 1 mm Aluminium sheets so I'll have a go at this this.

If I can't bend that then I'll try 0.5 mm sheet.

By the way, the device to bend metal is called a Brake.
If this approach doesn't work then there is another plan to use 10 mm blocks and then drill and tap holes in there to hold it together.

It's been interesting looking at how to solve problems along the way.