In addition to RyanDev's excellent suggestion, it's also worth noting that if you use this SQL:
SELECT max( enquiry_job_number ) FROM EnquiryJobs;
You cannot access the data from a DataReader like this:
SQLiteDataReader.GetOrdinal( columnName )
Becaus eyou haven;'t given the data column a name at all.
In future, try naming the column when you return it:
SELECT max( enquiry_job_number ) AS MaxJobNo FROM EnquiryJobs;
And you can access the column by name:
int max = (int) myDataReader["MaxJobNo"];