Huge thanks for all the sage advice here. Much appreciated.
In case anybody in the future is interested, I went with BlueCol Blue Anti-freeze and distilled water (after emptying and flushing out the horrible modern OAT stuff!)
All good now, runs great, no leaks, stays cool!
Many thanks
BTW
I just posted another little query about radiator mountings. Didnt want to mix up the posties (would that be a case of Mixomaposties? 
) so posted it separately here
If anybody can help with that, much appreciated.
Thanks again for the help
Mark