I have read various of the forum entries and I understand that I can set up a Master site and multiple slave sites which would all share a single VM installation - users / categories / stock.

What I would like to know is whether the shopping cart is restricted to just one of the sites at a time (i.e. order must be completed on only one site) or is it possible to retain a cart across multiple sites?

With VM, you have either the choice to share only the Users or share the whole VM.

Becarefull that VM store the shopping chart into the session and not in the DB and therefore when you share the Users, this also share the sessions.
So if keep the VM independent except the users, this means that you could have inconsistencies when you store something in the shopping chart and that after you change of website.
ie. Suppose that you have a product ID 1 in site 1 with label A and that the product ID 1 in site 2 has the label B (with price B ), depending where the users will check-out, it may be order the product 1 label A or label B depending if it check-out on site 1 or Site 2.

So yes the cart can be retained accross website when you have websites on the same domain (ie. subdomain or subdirectories) because they will share the same session.