A wise man's opinion (thank you @EUstaran):
It's really very simple. The key thing is not to make assumptions. You might assume previous consent is OK - but how did you gain that consent? Does it meet the standards as outlined by the ICO?
At conferences & everywhere people ask if previous consents need to be renewed under #GDPR. My take: Assess whether you need it. If you think so, think again. If you still think so, check how you got it. If it didn't meet GDPR standards, it needs to be renewed