Well, this is what i ended up doing
I took the bottom plug out of the rad and took the metal cap off and let it drain
Then I opened up the bleed screw from the hose towards the back of the engine
I then refilled the rad whilst squeezing the rad inlet hose until full, then input the bleed screw on and the cap on the rad
Started the engine, let it get up to temp, turned it off, topped it up
All is well
Maybe I just got lucky
I am going to do the same 3 more times, that way the majority of the coolant will be new by the end of it