I couldn't get this DB lock working over the fork, so I've gone a different route:

• Fork
• Use Proc::PID::File to check that this job isn't running.
• If it is:
  • Increment the number of attempts to start the job in the database
  • Select the no of attempts from the DB - complain if greater than X
  • Exit
• If it isn't running:
  • Reset the no of attempts in the DB to zero
  • Run the job

This way I avoid locking the DB at all.

Perrin, thanks for your help