I found this a short time after posting:
http://www.uboataces.com/boa-uboat-end.shtml
It mentions that the "final" message was sent on May 4th 1945 at 3.14pm, so it pre- dates the final order to the armed forces by 5 days.
I can't find any references which state how the order was issued, but I can only assume it must have been radioed otherwise who would have gotten the message?