In reply to RobScotland:
I use an MSR AC bivy, which has a bug net and cowl. I really like it - its also much bigger than most bivi's so feels very spacious.
Its one limitation is that the bug net portion can't be sealed so in really squally weather you may get some rain in.
Breathability isn't as good as the military Gore-Tex bivis but its not bad, particularly if its windy and/or you try to make sure you breathe out through the bug net.
My son uses an MSR Twilight bivi, which is smaller (its a pretty standard size so its not "small"), lighter and more breathable than the MSR with bug net and zippable cowl. Its not got taped seams but is very waterproof and hes not had any problems with it, although he's not used it in torrential rain.