Hour and a half is loads.
Have lots of games though, and a schedule up on the wall (control freak emoticon), with timings (allow only about 4 mins per game, but you will be lucky with some of them).
What's the time Mr Wolf still good at this age. Do you remember it? Someone is the wolf, faces the wall. All others start at the other end. 'What's the time Mr Wolf' - '12 o clock' or whatever - 12 steps up the hall. Eventually mr Wolf says Dinnertime and chases everyone. If the askers reach the wolf first, they have won.
Non-eco but excellent - everyone has to stand up when the lights are on and sit down when they are off (or vice versa). Hilarity ensues.
Warning - For pin the tail on the donkey with 20 children you will need AT LEAST 3 adults - 1 to marshal the queue, one to administer blindfold and wrangle child/tail, and one to mark where they've put the tail and write initials next to it. Unfortunately at 5 they will notice if you cheat too blatantly.
Put up a play tent (good idea anyway). If things get scratchy, get everyone into the tent, turn most of the lights off and read them a story, while another adult does sound-effects (not a scary one, just general sound effects, animal noises or whatever).
Pinata. Another queue one, and you still need the 3 adults to stop them whacking each other, but it's amazingly popular. The sweets from the pinata also deal with party bags.
If you do go for party bags, Toys R Us do 5 whoopee cushions for a couple of quid - a real winner at ds's 5th.