Namespace error

Hi guys,
I'm getting an error:
The type or namespace name 'DefaultLayoutMain' does not exist in the namespace 'StockHill.Web.MasterPages' (are you missing an assembly reference?)
and (I think related) error:
'object' does not contain a definition for 'Master'


This is my code:
public new StockHill.Web.MasterPages.DefaultLayoutMain Master {
            get {
                return ((StockHill.Web.MasterPages.DefaultLayoutMain)(base.Master));
            }


Any idea what I've done wrong?
Many thanks

...