Battery problems

I need to change the battery on my 300TDI Discovery - only thing is, I don't have the radio code to re-enter when the battery is re-connected. As I listen to the radio on long journeys to drown out the droan of the lump in the front, I could really do with finding a way of swapping batterys without loosing power to the vehicle - Any ideas guys?
I'd be obliged for any help

get a jump pack and connect it to the terminals before you remove the battery

Thanks for that, don't have a jump pack, but suppose could use our other car (focus) to do the same thing?

yup, as long as the power is on the terminals you wil keep the code

Thanks very much for that.