What is the best procedure when replacing the 3-port mid position valve?

I need to replace the 3-port mid position valve on a Systemate III. I have already replaced 3-port valves, but never on a Systemate III.

1) As it is connected on 28mm pipes, I understand that the 3-port valve’s part number is XG142. Has it got to be an original part from Gledhill or are there compatible 3-port valves available from plumber’s merchants?

2) What is the best step-by-step procedure for draining the system, replacing the valve, refilling the heating system and checking that the valve is working properly? I want to avoid damaging other elements of the system and would like to know if there are things specific to Systemate III that I need to look out for.

Many thanks for your help.