I've bought two Mothercare Cot beds from ebay for my twins, both look in excellent condition, I am not concerned about the cots themselves.
In the instructions for the cots it advises to buy 140x70cm cot mattresses and this is the standard size that Mothercare sell. However I had done lots research and finally plumped on some mattresses from Argos, which even better turn out to be reduced in price...bingo! Problem is they are 140x69cm.
Would that 1cm (I.e. 1/2cm either side) be of cause for concern for you?
If I went to Mothercare for the exact stated size I'd have to compromise on the type of mattress I could afford, whereas I can have exactly what I researched and felt was right for us if I go with Argos.