Oops, terribly sorry. It seems that the SQLite DB assumes nullability unless otherwise stated - unlike TSQL which requires it in the create statement.
In fact, I found hidden away in a sub-method that the particular property had a not null set on it in the mapping. Removing this sorted the problem.
Thanks for posting about conventions though, I'll have a look as I have another issue that they might sort.
Cheers
Stu