Our WHs must not be reading the same manual.I've raised and bred pure Welsh Harlequins for over a decade and have always had them hatch out on or around Day 23, with hatch rates running 85-100%. I have a friend who lives in Wales and his WHs and Khaki's all hatch out around Day 23 as well. If you look up WHs or Khaki Campbells in the UK, you will find that they hatch out anywhere from Day 23 to Day 26.
Strange, mine have always hatched on day 28, maybe day 27 or day 26. Are yours fairly small ducks? I've never even had Calls hatch before day 26.