Need to NAT to a host on a different interface over a Site to Site VPN

Hi

Hoping somebody can help me as I'm going round in circles. We recently replaced a Cisco Firewall. A support company that looks after one of the servers on subnet 10.70.0.* had a site to site VPN to a management VLAN of 10.7.1.*. From there, they would hit a NAT IP, which would translate to the the server IP in the 10.70.0.* network (both forwards and reverse).

The Site to Site VPN to the management VLAN is working fine, but I just cannot get my head around setting up the NAT in the Fortigate. Tried as if it were an external NAT, and that didn't work. Tried adding the server VLAN to the VPN and doing a NAT (even a Zone) and that didn't work. Feel like I'm missing something, but can't find a guide on how to do this. Below is a diagram of what I'm trying to do if it helps?

Many thanks in advance. Dan