Can anyone tell me how to lengthen the straps on the Maclaren XT. The straps are now getting really tight (dd only 14 weeks) but I can't see how you lengthen them - there is some extra strap around the buckle but I can't get this to budge and I can't see any extra strap at top of the two top straps or the bottom of the bottom strap. Am I being really dense?