Thanks for the help everyone, this is all really unclear from BA, but I guess in their de3fense, trying to encapsulate all the scenarios in a single note is difficult as respondents here have amply demonstrated.
Anyway, looks like I retain gold until March 2021 irrespective of any tier points collection (even zero), and will see if I need the full or the reduced amount in 2021-2022 - I will park that one for now although advice here seems to indicate that the concensus is that I'll need the reduced total.
Thanks again, very much appreciated.
Best,
CR