Data in subform combo box not determined by main form selection

[David Ewer]

I have created a form with two subforms based on the data from three tables.
I have a Customers table, a Customers Site table, and a Site Details table.

If I select a customer from my main form then I can select a site from a
short list of sites that belong to that customer. If I then select a site
from the first subform then details about that site appear in the second
subform. So far so good.

What I want to do in my second subform is present the list of customer sites
as a combo box rather than navigate left and right through records.

Is that possible and how do I do it?

Thanks

David