Amazing! Thank you so much Joachim!
I inherited the doc from an InDesign doc that was imported - so the choice wasn't actively made by me, but by previous author. I think the purpose of lifting the heading, at least, is to introduce a slightly less "aligned", slightly more "wild" look to the document, but why the Copy style was set to override the baseline grid I cannot explain, as the override is uniform.
I think what I need to do is reconstruct the document from scratch and then know, and feel comfortable, about each choice made, rather than navigating my way through it.
Once again, thank you so much for the rapid response. Haven't been a Affinity user for long, but I am slightly developing a crush on Publisher, Photo and Designer right now A super helpful community is obviously a key part of it, so thank you again for the warm welcome and help.