DONT MIX THE COOLANTS.
 
mixed coolants can solidify and clog pipes and channels....
 
also make sure you read the instructions on bleeding and flushing the system... no shortcuts.

Um, when you get the toyota coolant, the side of the container states to dilute with distilled water to a certain ratio between 1:3 and 1:2 if I remember correctly. You certainly don't mix one coolant brand with another!

Hi All,
Thanks for all of the responses & information.
Have done the job, replaced the Thermostat. Old Thermostat had movement just flopping around loose.
Internals all looked nice & clean.
Filled & flushed with distilled water twice to remove as much green coolant as possible.
Followed the procedure. Had no real difficulty, just a little awkward doing it on stands in the garage.
Idling o.k. now. Will keep a close eye on coolant level for the next week or so.
Thanks again.