Obscure error message from 2.25.0

[Graham King]

I've just run convert-ly on a 600+ line file and its \include files, from 
2.19.40 to 2.25.0.  Nothing seemed to change apart from the \version statement, 
but on trying to compile the file, I get:

Processing `/tmp/....ly'
Parsing...ERROR: In procedure %resolve-variable:
Unbound variable: %
Exited with return code 1.

Does this point to an obvious problem?
Before I start laboriously bisecting the file, is there a way to get an error 
message that points to one or more lines of code in the file?

Grateful for any help

-- Graham