I bought a 1 man Shakespear Cypry bivvy this year and have found it to be excellent. Didn't let the rain in when it lashed it down. You can get an additional storm groundsheet for about 15 for extra protection. You can also get storm fly sheets as well, but I don't know how that differs from the supplied fly sheet. (Thicker maybe???).
If you look around, you can pick one up for about 65 and they've also got the zip up porch so you can use your cookers out of the wind & rain.
Bivvy wise mine's a Rod Hutchinson Geo 2.2 and an excellent bivvy. Also check out the Shakespeare Cypry Excel as beebs has said.
One thing to look for when buying one is the ease of putting up. Some can be an absolute nightmare to erect on your own. For one nighters you could also look at the 'overwraps' that can be bought to go over 45 and 50 inch brollies
I can vouch for Dave's being big enough, at the Trent Fish-in he had 35 sheep in his and there was still room for his bed and coloured telly. I must point out though, that the sheep were only in there to protect themselves from Phil, who seemed to take an unatural interest in them all weekend.