I apologize if I haven't responded to you, if I'm the Stuart you are referring to. I have missed emails in the noise, get tons of email. You could be sending messages to an old email address "firstname.lastname@example.org" - this service has been discontinued. Use email@example.com
to drop me a line.
Terry's response and advice is of course sound. I rarely take my masters swimmers above 200m, and do short focused repeats - each week is a new theme (hip drive, connecting to the core, lengthening the vessel, race prep, etc) . But also, have swimmers get comfortable swimming in a crowd, and close contact both in pool and open water. I suspect as Terry noted, you blew up in the first 200 yards of your race due to not being prepared for the environment at the start of the swim, and everything fell apart. This is very common and see at least 10% of swimmers in each wave of a triathlon distressed at first buoy trying to recover. So I suspect it was more environment, and not because you didn't swim longer single sets.
In any case, feel free to drop me a line at firstname.lastname@example.org