Here in Scotland we were issued a Safety Action Notice on 25 Feb 2004 SAN(SC)04/06 BED RAILS (TWIN-BAR DESIGNS): RISK OF ENTRAPMENT AND ASPHYXIATION.

It summarises that:
"Bed rails of a twin-bar design with a gap of more than 120mm may cause a risk of patient entrapment and asphyxiation. Implicated bed rails should be removed and destroyed.
Current guidelines based on the relevant published European & British Standards reccomend that the gap shouldn't exceed 120mm. And the gap between the mattress platform and the lower bed rail also should not exceed 120mm"

It gives reference to
DB2001(04), Advice on the Safe Use of Bed Rails, Medical Devices Agency, July 2001.

The Safety Action Notice can be viewed on
www.show.scot.nhs.uk/shs/hazards_safety/adverse
Hope this is of some use!