I've just heard back from the booking officer at the town hall (an employee, not a councillor) with a quote. Obviously if the Lord Mayor could help us then these figures would be revised downwards.
First bit of news is that the weekend of the 7th of August is booked already (as is the 3rd of July), so we'd probably want to have it at the very end of July instead.
Second bit of news is that there's a 20% discount if we book all of the public rooms, so on a 6 hour per day basis he's quoting £10569 (compared to £2844 for just the main hall).
The final, and perhaps most important thing to note is that, and I quote: "If your organisation is a registered charity I may be able to offer up to 50% discount on the standard fee." I will e-mail back to see if he'll accept Wikimedia's not for profit status, but I very much doubt he will. Is this a sufficient motivation to do everything conceivably possible to hurry our application for charitable status? Or perhaps to just commit to creating a new charity solely for the running of Wikimania 2010.
Tom