We found exactly the same with our Mothercare cot. I guess you could wedge the castors so it's hard up against your bed, and fill in the gap with towels. I have to say that we abandoned the idea of having it as a bedside cot within a couple of nights, and got a bed rail instead so DS could sleep in bed with us.
If the cot arrived less than 7 days you can return it under the distance-selling regulations without having to give a reason, as long as you still have all the packaging.