Remote subnet as IPSEC local network?

Hi

I have a VPN to a customer, due to a subnet clash with one of their ranges my end is NAT'd, this works fine. At their end their local network in the IPSEC config is a defined network object (not an interface) 172.18.1.0, this subnet is visible from their 172.16.0.x subnet (their internal interface). The VPN connects but I'm unable to ping anything on the 172.18.1.x network

Thanks in advance