Event registration fatal error since v5.0.1
The two versions Stuart mentions are different extensions. 

https://civicrm.org/extensions/sparkpost-civicrm-integration is the PESC extension from https://github.com/proexchange/com.pesc.sparkpost and the replacement used by Stuart is the Cividesk extension.

I had the same problem experienced by Stuart with the PESC version on a D6 system running CiviCRM 5.0.0. I seem to have fixed it using Stuart's sleuthing - I simply edited the sparkpost.php file and replaced source_record_id with source_contact_id

Everything seems to now work. (I'll report back if I find other problems).

UPDATE: The latest version 1.3.7 of com.pesc.sparkpost fixes the problem.