... with a dish with a 8 degree beamwidth. There is a strong null even when right overhead, if the tracking system is way off. The maximum range I can receive a TV signal is 800Km to the ISS.

As usual, I shot from the hip and pontificated about the disadvantage of an XY mount when in general they are ideal overhead if you don't need the two horizon points that are problematic. Now that I understand that you are only targeting the high elevation portion of all passes, then you are absolutely correct, that the XY will be better since it gives you un-hindered overhead high elevation successs without the zenith keyhole.

The aim is to use the entire pass without a gap in the middle, and then hand over to another European station to continue the contact.

AMEN. Great plan and I agree the XY is ideal since you don't care about the low E/W horizon.

With 800KM range I will not be able to pick up the signal from ISS at

low elevations.

Would you recommend XY or ordinary AZ/EL?

You are right to choose the XY for this mission. Thanks for hanging in there. Devil is always in the details.

Bob, WB4APR