Found this with Google:
* Check your cot bed conforms with the BS EN 716-1 safety code, which ensures the cot is deep enough to prevent your baby from climbing out.
* Ensure the mattress fits the cot bed with no gaps.
* Make sure the bars of the cot bed are no more than 45mm to 65mm apart. To test this take a fizzy drink can with you when shopping, this should not fit through the gaps.
I think the current standard is BS EN 716 2008
Hope that helps,
Simon