How do I reference a cell in another sheet using a value from the active sheet

I want to reference cells in another sheet, I know how to reference them directly but I need to reference them from a value that points to the cell that is in the active sheet. 

I know this doesn't work but gives you the idea. 

=sheet1! (valve in active sheet C3)

Where cell C3 is the cell number I want to access in sheet1. 

Ie

I have a column (B) of numbers (eg 1-999)

And column (C) with the cell (eg ="N" & text(C1, 0) giving text N1 to N999

I've even got the whole lot in a cell that reads =sheet! N1 ... Etc, exactly as the direct reference, but it won't return the value. 

Hope you can help