Tara I was actually just about to email you and tell you this same thing. lol
We have a Pottery Barn crib and a mattress from Babyshop, and it fits great. The Mothercare ones won't fit, they are longer and skinnier. Be careful though because Babyshop caries different sizes... I think the size you will need is 130 x 70... It's something like that, I could be off by a few cm...
I was just at Pottery Barn today and was told that the only other store to sell mattresses and fitted crib sheets that will fit PB mattresses, is Babyshop. Reason being, PB sells American sized mattresses, and everywhere is UK sized.
I know you are asking about Mothercare but I have a Mamas and Papas mattress also and the sheets from PB are very tight, quite difficult to get on. Still doable but not easy so I ask my husband to do it whenever I can. The dimensions are different so I presume Mothercare would be different to PB too. The PB sheets are lurvely though so I buy them anyway!
Does anyone know if mothercare cot mattress would fit the potterybarn cot? I prefer the cots in potterybarn as I am looking for dark wood but prefer the mattresses from mothercare. They also seem to have more choice of mattress in mothercare...
Thanks