You don't say if the oven control 'stat has been changed or not. If it's a fan cooker, the stat comes in 2 parts - the fan selector and the main oven thermostat. The oven section very often has 3 terminals - 1 single and 2 linked. Without having a wiring diagram, I'm going on it being an older type of oven 'stat. It's possible that the line (live) and load wires have been put on the wrong terminals. The live wire should be on the single tag, and the load to the oven element and the neon wire should be on the double tag. I'm assuming that when the 'stat is set to "off" that you're not getting any heat in the oven - if you are getting any heat, or if the neon goes off after a while, you'll probably need to replace the stat, although some are adjustable (and there is a cheat on that as well.....!)
Chris, May 2009