Happily everyone's right (probably). PPL coursebooks refer to the figure as 1.5 Deg C per 1000 ft, for the ATPL writtens (and I would have presumed the CPLs) it's referred to as 1.8 deg. Still doesn't explain why WBs notes say 1.5 though. Maybe it is 1.5 for the CPL?
FI course coming up and reviewing the PPL material again. :Insert suitable smiley: