Fortinet Routing Issue

I have a Fortinet Fortigate-100A and believe this is stopping access to
one of our remote networks (when connecting onto the network via a
Cisco VPN).

Our VPN comes in via ADSL (Cisco 800 modem), is connected to the Cisco
Pix which is then connected to the Fortinet which in turn connects to
the local lan in the office. From this lan are 2 further Cisco
connections out to different companies.

We've added a static route to the company PCs and they can all ping the
remote network OK. When connected via the VPN I can only ping as far as
the Cisco 1800 (10.226.142.251) on the way out.

I've added the same static route to the Fortigate (I only have the web
interface)

Another remote network which is configured in the same way is accessed
fine and routes out via a Cisco 500 (10.226.142.1).

Any ideas how to connect to this other network via the .251 Cisco. Is
it the Fortigate or the Cisco blocking the VPN traffic?

Thanks
Martin