You shouldn't use a standard household timer with HID lighting, particularly if your ballast is the magnetic type. It will stick in the on position sooner or later, due to the poor quality switching components that are used in the average timer, and the fact that mag ballasts have a much higher inductive load on startup, compared to their stated wattage. At the very least, a stuck timer will disrupt light cycles, and has the potential to be more hazardous (fires etc).
I recommend you look for a heavy duty timer which is rated for HID use. The cheapest one that I know of is the "plug and grow 22A heavy duty timer", which is good for up to a 600 watt magnetic HID. These are usually around £15. A slightly more expensive solution, available in most grow shops, is a contactor relay setup, like the maxiswitch light controller. These devices use a relay to switch on your light, which prevents the initial startup surge from passing through the timer and welding the contacts together. Expect to pay around £40 for the 2-way version, with a grasslin timer included.
People may say "but i've used a standard timer for ages and never had a problem", but it doesn't change the fact that to do so is unsafe, and at the very least you risk interupting your light schedule when the timer sticks. The money spent on a proper timer or relay is money well spent, particularly given the shit quality of the average timer sold in the UK.
