A brief summary along the lines of Seth:
Algae, fungi, lichens, bryophytes, cnidarians, molluscs, worms, myriapods, crustaceans, springtails, other small things - these groups don't really rock my boat, and I don't know much about them - 48 species recorded so far, but probably won't increase that much.
Vascular plants - 102 species recorded, be lucky to get to 200, not a very rich square and can't get to the more interesting areas.
Arachnids - 38 species, all from my garden, not sure how many extra are in the wider area, but could get to 75?
Odonata/Orthopteriods - only one so far, but could possibly get to 25-30.
Hemiptera - 21 so far, but possibly another 100 if I put my mind to it.
Hymenoptera - 10 so far, but if I push with bees, wasps (including some ichneumonids) and sawflies I think an extra 100 (or even 150?) might be possible.
Coleoptera - 87 so far (from my garden). If it's a good season for migrating/dispersing beetles coming to light, it's not impossible that I could get another 200 species, but could be considerably less.
Diptera - 60 species so far. This group is a bit of an unknown quantity as it's only over the past 6 months that I've become confident about some of the groups, so I should get far more this year than previous years. I've seen 330 species in the square since I moved here, but it's likely that this will increase significantly during the year. Perhaps another 200-300 species?
Butterfles/Moths - 24 moths so far. Should get 20+ butterflies. Moths more difficult to predict. A bad year might be 350 species, a good year 450 species. I'm away for nearly half of June, which is not a good move and could miss lots of species, especially if it's warm whilst I'm away.
Birds - 76 species so far. Should get 120+ species I would think.
Mammals - 10 species so far, should see a few more.
New stuff in the last few days: