Hi Gwallogwyn
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.
S. deltoidea is a rare species and your plant is precious.
The plant has grown 4 light-textured flowers (unusual), and is not able to produce the final most important feature ::
a strong fragrance. Moreover, it has made a second flowering in a relatively short time ...
From this 'circumstantial evidence' I believe that your plant may be under duress
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, and with due respect
I suggest that you consider -beyond a few weeks of hot weather- whether there might be a need to improve
it's general cultivation conditions ...
Regards,
Manfred.