UK police drama where a serial killer unwittingly reveals his victims' bodies by...

I'm trying to identify an episode of a UK police drama, possibly Morse or Frost. From around 2006.

Some of a serial killer's victims were never found. Years later, the killer is taken out of prison to assist in the hunt for the bodies. On the way back to prison, he stops to urinate.

The detective figures out that the spot where he urinates is where the bodies are concealed.

You're describing the pilot episode of Cold Blood (2005). The murderer gives away the location of his victim when he asks to stop the car, ostensibly so he can urinate.