How do I access trunk entry into my 2004 Jaguar xj8 when now it has gone into valet lockout?

Unlock the trunk, using the master (black) key. Earlier cars had the keyhole in the model badge on the back of the trunk, later cars have the key hole up inside the chrome handle (under the trunk button).
You can get a new master key cut from your valet key, or from your key code see http://www.triple-c.com/How_To_Read_Jaguar_Tibbe_8_Cut_Key_Codes.cfm for details, or if you look at the tip of the keys shown at this web page, and compare with your valet key, you will notice that with careful filing, the tip of the valet key can be filed to turn it into a master key.

I believe that this is to protect valuables left in the boot when parking the vehicle. I read here that there is access some where in/ near the glove box but I cannot remember exactly what it was . Surf the net under Jaguar valet problems and you may get the answer.