>Hi Viv, that's exactly the scenario - I'll try what you suggest ( casting ) because it would be a lot easier to maintain, I'm trying ( in vain so far ) to get the companies that supply us with these shite spreadsheets to make more of an effort to send them in a consistent format which would make life a lot easier. Thanks very much for your help.

I don't see how that would work. To cast to the derived class you have to know which class is in List.
If you don't know you can't cast.
If you do know then you may as well type the list to the derived class in the first place...