There is a great bit of kit I am using in a conversion to do just this - called a webbrick. have a look at http://www.o2m8.com/ this thing has built in schedules, web configurable or you could use cron with python to control from your computer. It will fit in a standard consumer unit (din mountable) an is a doddle to set up initially, as well as being flexible if you want to have a more complex configuration.