Very clearly set out in this article in the guardian -
If half of cot deaths occurred whilst the child was in the bed, then half occurred whilst the child was in a cot. Therefore, co-sleeping in a bed presents no increased risk, (where no drugs or drink have been taken)
In actual fact, the 54% of cot deaths that occurred whilst co-sleeping include those where parent was sleeping with the child on a sofa, and those where parents were under influence of drink or drugs at the time.