My first-edition copy of AP837 is amended up to AL5 of Feb '44. It makes no reference to the para that AJ outlines, so it suggests that the provision of cloth through stores was made "official" sometime after this.
Yesterday I saw a painting of an Air Marshal wearing such a blouse with the stepped collar, which was being worn by 1946. Other photos seen during the day also showed period Equestrian Team wear which included a light coloured (on bw photo) breeches being worn with standard SD tunic. I can't recall the date but it would have been 50s/60s. This would fit in with the cotton twill breeches on ebay - so essentially they are sportswear and not parade wear.
Having a read through the same regs bring up some interesting points - for example officers above Gp Capt (inc) being allowed to wear cloth peaked SD caps as an alternative to the leather/gold braided peaks (A.380/43).