Have a look at this page, it'll tell you what you need to fix this:
Essentially, the heap is the amount of working memory you allocate to Neo4j. If you're running inside of Neo4j Desktop, you probably have a fairly low amount of memory allocated, and the error just means you're trying to do something too big to fit into your memory. So in general you should increase the size of your heap you allow to Neo4j. You can do that with the configuration settings on the page above.
Thank you both very much. Strangely I had tried this, but the settings had reverted to the default ones for some reason, so I assumed that it was the incorrect approach. However, I upped the dbms.memory.heap.max_size to 5Gb this morning and it seems to have worked! So thank you. I'm now going to run the enrichment and see if that also works. Thanks very very much!