I used a coffee filter to filter it SLOWLY back in to the overflow.
I had plenty as I had shortened the rear heater hoses and recovered the coolant from those hoses.

I feel the coolant I should use when I do change coolant is probably
Penrith 7 year 450,000 km Green Coolant.
probably need about 10 - 15 litres so as one 5l makes 10 litres but can have it more concentrated I maybe need 5L and 1L and then see if more is needed.

I am pretty sure I don't need to flush it thoroughly as this is probably similar enough to what is currently in the system.
Probably needs doing in a few years.

Here is inför about coolant
https://www.penriteoil.com.au/tech_pdfs_new/Nov2015/Coolants.pdf