NameError - any pointer on how to track down problem

I'm trying to put Spree 0.30.1 on Heroku and I'm getting the following
error message (from the heroku logs);

NameError (undefined local variable or method 'auth_user' for
#<ProductsController:0x2b4ee9fb07f8>):

I've tried Google and the Spree Google Group but I'm no nearer to an
answer. Can anyone point me in the right direction
(or any direction)?

Ewen

1. Does your app run locally in production mode?
2. Is whatever supplies 'auth_user' specified in your .gems file?
3. Does your Heroku DB match your local DB, either through having
    run all migrations or doing `heroku db:push` ?