The hearts' specialist, Francis Ow passed away on June 29th 2018 after a long struggle against cancer.
You can read or leave an homage there:

In its great kindness, Francis freely gave his booklet “More Origami Hearts 4U”. It is a collection of his origami heart's models by Francis and “Heart Box” designed by Edwin Corrie. This booklet is a PDF that can only be downloaded from this website.

Disclaimer: You are free to teach any of those models without asking for permission, but any other usage (including the diffusion of individual diagrams) have to come from an agreement with Nick Robinson who is in charge of these questions.