The Feb 2007 Department of Health leaflet recommends mattress with a waterproof cover but doesn't mention new ones: www.dh.gov.uk/en/Publicationsandstatistics/Publications/PublicationsPolicyAndGuidance/DH_4123625 and even the FSIDS site doesn't mention it in their main guidelines www.fsid.org.uk/babycare.html. The place where I found the info about new mattresses on the FSIDS site also has mothercare branding on it - so I think there's a bit of commercial interest there! Anyways, mine doesn't have any tears or holes so I think we're going to use it again.
Ahh - a bit of googling and I've found the initial source I think!
"A new finding was an increased risk of dying of the syndrome for infants who slept at night on a mattress previously used by another infant or adult (odds ratio 2.51 (1.39 to 4.52)). However, this increased risk was not established for mattresses totally covered by polyvinyl chloride."
Case-control study of sudden infant death syndrome in Scotland, 1992-5, BMJ 1997;314:1516.
A 2002 study says you need a new mattress but doesn't distinguish between waterproof or not waterproof mattresses:
Used infant mattresses and sudden infant death syndrome in Scotland: case-control study. BMJ 2002;325:1007
An interesting comment on the above study which suggested a link can be found at:
www.bmj.com/cgi/content/full/325/7371/981
Anyways, after wasting my morning with google instead of working, I think we'll use the old mattress!