I'm not in Wigan but ours was listed as a '27 month check' and the questionnaire they sent indicated it should be done between 25 1/2 months and 29 months so they're not late, just a misleading name.
You can actually download a copy of the paperwork it's called 'ages and stages questionnaire 27 months'. You don't need to, but it might set your mind at rest to see what they are looking for and whether you need to be concerned about the delay.