I was also under the impression that Dicks didn't have the entire plot worked out, but he had something like 40 pages of script because Tom Baker wouldn't commit until he saw a script. Then Tom passed. There may be correspondence in Dicks' papers that Guerrier is drawing from.I thought it barely got along, they were in such a rush that they didn’t work anything full up and it all changed once Tom isn’t showing up.
I'm curious if the role of the Castellan will be filled by Colin Baker's Maxil from "Arc of Infinity," which I believe was the original plan.