Page 262 What are Supercooled Water Droplets (SWD's)?
Figure stated in book is that supercooled water droplets can occur down to -45 degrees celcius.
ATPL study books as well as for CAA exams have taught me that SWD's occur up to -40 degrees celcius. Other independent meteorology web sites and in particular, the official "Bergeron Process" states that SWD's occur up to -40 degrees celcius also.
Boeing Operation techniques for use of Anti Ice state that at -40 degrees and below does not require anti ice.
So I think Bristow though correct with the explanation, is incorrect with the temperature figure he states.