Locking records on a form

I have implemented a form in C# containing 'detail' fields dragged from the DataSet view of one of my Access 2003 tables. This process produced data bound fields with a navbar and display my data fine.

Now I want to use record locking (and not transactions - which I am not sure Access supports anyway) when this form is open on a record.

I have set the 'Default record locking' on the database to 'Edited record', the 'Default open mode' to 'shared'.

If I run two instances of the app and open the same record on the same form I get no locking errors and can modify data. It is only when I come to save the second form I get a concurrency error, which is not what i want.

how can I lock this record/detect a problem before if a second user opens the same record?

Thanks