OK Simon,

The first thing I would do is make life easy for myself and get everyone on the same class of address.

ie. Get the 'Remote Worker' on a Class B Address, we work with Class A Addresses

So our office is on 192.168.254.0 range

Remote Office 1 is on 192.168.1.0 range
Remote Office 2 is on 192.168.2.0 range
and so on

We have to remote offices calling our office!

now the important bit if you have all the usernames/passwords and authentication setup is at the bottom of the page

For all the Remote Offices it's the same: -
My WAN IP 0.0.0.0
Remote Gateway IP 0.0.0.0
Remote Network IP 192.168.254.0
Remote Network Mask: 255.255.555.0
RIP Direction TX/RX Both
For NAT Operation Private IP
Change Default Route UNCHECKED

On the Head Office Router the settings for Remote Office 1 is: -
My WAN IP 0.0.0.0
Remote Gateway IP 0.0.0.0
Remote Network IP 192.168.1.0
Remote Network Mask: 255.255.555.0
RIP Direction TX/RX Both
For NAT Operation Private IP
Change Default Route UNCHECKED

and that's it.......