I have a similar problem (with the timer), but unfortunately I do not have a matching Thorn remote control. On the assumption that you do, according to the manual, carry out the following steps:
1. On your remote control, press Menu Select (the Menu Select symbol looks like a a small white square within a black square - or it could be the inverse, as seen on the TV's front facia panel of control buttons! Because the child lock is set, these TV buttons are inoperative).
2. This should display 6 Menu options at the bottom of the TV screen as little symbols: a key (the child lock), a horizontal bar, a forward arrow, a double forward arrow, a clock and an X. These can be operated upon by the numbers 1-6 on the remote control.
3. To set the Child Lock, you press and hold the number 1 until the red vertical key appears at the top right of the screen.
4. Conversely, if the child lock is already set, repeating Step 3 should remove the key. It's a toggle.
Best of luck.
Lorraine
July 2006