Halfords are talking rubbish. (So not change there then.) You do not need any security codes to change the battery on a Rover.
Buy a CR2032 battery. (It will cost a fortune at Halfords BTW so you may want to shop around.) Assuming a two-button plip (fob). Prise it open. Remove the battery. Now press and hold each button in turn for five seconds. Now fit the new battery and press the plip back together. Unlock the cart with your key. Now press the lock button at least four times.
If the plip still does not work it will need synchronising to the car and this can only be done with a diagnostic computer.
If you have a single-button plip. The battery procedure is similar but if it has lost sync you can DIY it. (In the handbook.)
NONE of the above needs any security codes.
T
March 2009