http://tube.tfl.gov.uk/charter/default.asp

"Eligibility for customer refunds

Customers who are delayed by 15 minutes or more on any journey as a
result of the closure of the Northern line will be able to claim
refunds as follows:

-Only one refund application per passenger per week is required
-All single/return/daily ticket customers can claim their refunds
through the normal Customer Charter process. Customer Charter forms
can be obtained at any Underground station.
-Monthly/Annual ticket-holders from stations north of Camden, can make
a single application via an e-form on the Tube website. There is no
need to attach your ticket.
-7-day ticket-holder from stations north of Camden can also make a
single application but should use the normal Customer Charter form.
Copies of tickets will need to be attached to this form.
-7 Day/Monthly/Annual ticket-holders from south of Camden should use
the normal Customer Charter process for each delayed trip."
[ends]