Does anyone know whether a Quinny Freestyle footmuff is compatible with a Quinny Formula? Also, at what age would you generally stop using a footmuff? DS is 18 months, and I'm not sure he'd like the Quinny Formula footmuff which ressembles a mummy sleeping bag, but might prefer the Freestyle one which just comes up past the waist - has anyone got either of these and what do you think of them? Are they worth the £30-£40? Questions, questions...