Therefore, best time is either July (yes I know it sounds odd) or during a
cold mid winter.

agreed, although i usually wait til aug/sept for formative pruning since
july has been so wet these last few years, and early autumn relatively warm
here in yorks. a couple of ornamental cherries i have here (p. incisa? not
sure) always flower in february ... and sometimes bring out the occasional
bumblebee ...

http://www.yeknom.karoo.net/P1010676.JPG

.... although the bee in the picture had failed to notice the looming
snowstorm this year.