Tested two approaches:
1) php template using a lot of php includes for all the shared content and unique content hard-coded
2) php template containing all the static content and then a php include for the unique information
Former has advantage of one copy of shared content, but Contribute can't make sense of what's left.
Latter works in Contribute, but has disadvantage that every page has hard-coded all the common stuff.
Installed DW to see if there were some way to create in DW a php page that gave me the advantages of only one shared source, but which appeared to Contribute in a way it could deal with it, no success.

So, I think I'll just have to go with a template for each section, each containing all the common code, with php includes for the nav and css specific to each section. Editing of common bits will have to be by multi-file search and replace.