http://www.sedgwickpark.com/index.php - the place looks really nice!
The event is from 17.30 - 10.30 (gates open 15.30) and the Big Band are on first (shame they're not on mid-evening, really).
The line-up also features the Bernardi Symphony Orchestra, Mid-Sussex Youth String Orchestra, Joni Fuller (winner of the 2005 Junior Eurovision Contest, apparently!

Take your own food and booze, or buy it on site.
Tickets are £20.00 from the Capitol Theatre (Horsham) Box Office :
http://www.thecapitolhorsham.com/contacts/contacts.asp
The evening is in aid of the National Defibrillator Scheme.