Extension does not work with CiviCRM Webforms with Contribution page

Created by: PJKerrigan

I am using civicrm_webform (7.x-4.19) in Drupal (7.52) and have a webform which uses a Contribution page on CiviCRM 4.7.30.

The GoCardless payment processor fails with the following notice:

Notice: Undefined index: description in CRM_Core_Payment_GoCardless->doTransferCheckoutWorker() (line 122 of /var/www/vhosts/my.site/dev/sites/all/extensions/uk.artfulrobot.civicrm.gocardless/CRM/Core/Payment/GoCardless.php).

I have tried to simply set the $params['description'] value by hard-coding it, but that led to the following error being displayed:

We can't load the requested web page. This page requires cookies to be enabled in your browser settings. Please check this setting and enable cookies (if they are not enabled). Then try again. If this error persists, contact the site administrator for assistance.

Error type: Could not find a valid session key.

I'm guessing that the description is used to create the session key, so changing it in CRM_Core_Payment_GoCardless::doTransferCheckoutWorker breaks it.

Does anyone have any ideas? Thanks.