Access method in master's master

Hi there,

I'm trying to access a method in a nested master, so far without much
success.

The structure is as follows:

'Main master' (containing public menu setting method) is master to 'Sub
master', both of which are at the site root. The 'Sub master' is master to
'Content page' in a folder.

Master.SetMenu("Products") - works if just used on a test content page at
the root, but...
Master.Master.SetMenu("Products") - fails with an error saying 'SetMenu' is
not a member of 'System.Web.UI.MasterPage'. I understand the error, but how
can I access a method in a master's master?

Thanks

John

PS - I'm using <%@ MasterType VirtualPath="~/Sub.master" %> (and
"~/Main.master") in the respective masters.