I'm putting a radiator in my conservatory so I can use it on the odd
day in the winter so I'm going to add Fernox alphi11 antifreeze to the
system. How much do I need to add?

Is the conservatory on its own zone with a programmable room thermostat?

Mine will be, so won't require antifreeze, as the programmable thermostat
has frost protection. It's pretty hard to freeze burst plastic pipe, too, so
it might be an idea to plumb in that, rather than copper.

Christian.