Thanks B.
I quoted from a medical website I came across when searching Mr Google for my own edification (have tried to find it again but can't soz) I had been confused about her symptoms, and the Dr's possible dx, and that seemed a concise summation to share.
MIL suffered with high bp for years. On meds since her early 30's. FIL died, and BP meds began to be reduced, and reduced...until they were finally withdrawn as her bp was being pushed too low.
Next step was diastolic dropping lower than systolic. This has been gradually falling, and now sits routinely below 60.
Fainting episodes are becoming more frequent, and are always related to standing too quickly or standing for a relativley prelonged period.
She has never sweated

and has had a periodic Parkinson like shake of her head for some years.
A definitive dx of Shy-Drager is difficult, but the deterioration of symptoms would tend to point that way.
Life expectancy once symptoms present is between 7-10 years, and as she is 75, even she is ok with this.