I spend so long grumbling over tricky procedures that people must wonder if I've full command of my fingers! Admittedly I've made plaster casts for relief tiles, but this four-part mould was a different order of difficulty. I'd just finished when a kind friend* gifted me Sasha Wardill's Slipcasting bible... so I could see what I should have done. Once glazed and fired, I'll be sure to post any results in a few weeks.
* Thanks again, Ben!