Okay, here are a few tips for doing some diy searching and booking, firstly you need to sort your flights out as these are what are in short supply. If you go to
http://www.skyscanner.net you will see a facility to search for flights, put where you want to fly to and where your flying from (all London in the boxes, then choose whole month for september. You will get a list of flights that are available on each of the days and if you click on them it gives you the prices.
Once you have sorted the flights have a look at some sites that do independent hotel bookings, two of my favorates are
http://www.alpharooms.com and
http://www.wintershunshineholidays.com but there are others available, again put in the details of where you want to go and hey presto you will get a list of accommodations available which can be sorted by price. On these sites you will have to put down a 10% deposit and pay the balance within 4 weeks of travelling.
dont forget to factor in your taxi cost from and to the airport, should be no more than £20 each way...ish
Good luck
by the way, the majority of clubs are not around San An but around Ibiza town area however its only a short bus or taxi ride to ibiza town and often or not a lot of the ticket sellers but on buses if you go to some of the preparties on the sunset strip in san An