Hi Mark,
The latest standard relay I've got on here happened over the weekend just gone, 15th Oct 2011...
http://www.dover.uk.com/channelswimming/swims/2011/10/15/2542/Eltham+Mermaids/
It's not yet been ratified, and I don't have the CSA list for this year, but I'm pretty sure it's the last.
The only other swims (of any type) that are equal/later are a solo swim on 15 Oct 1991, and another solo on 28 Oct 1979.