Ok, I have resolved the issue.
In frustration at not being able to fix the issue with 6.1, nothing I did made any difference to the result, and even building and stepping through the EF source accomplished little so I rolled back to EF 5.0 and rebuilt my code.
EF 5 immediately returned an error message identifying an issue in the schema definition, a view added by another developer that referenced system views was confusing my template, I fixed the issue and was back working in a matter of minutes.
The lesson learned here was, EF 6 needs some serious work on its error handling, as just returning a generic exception error left me chasing off in every direction, while the error message from EF 5 pointed me directly at the issue.