I currently have a BM 3.8 server configured with 2 NICs running HTTP proxy
only and is working fine.

Private 10.x.x.x
Public 195.x.x.x

We also a SonicWall firewall on the same public subnet (leased line internet
connection) with an ADSL backup.

Private 10.x.x.x
Public 195.x.x.x
ADSL 82.x.x.x

I want to configure the SonicWall to fail over to the ADSL if the leased
line goes down, but this is reasonably straight-forward. The problem is that
for the BM server to be able to utilise this WAN ISP fail-over, I need to
move it from being public facing to be behind the SonicWall firewall.

The only way I can see of doing this is to only use a single NIC on the BM
(basically have it on the LAN only) and set it's default route to be the
SonicWall but my question is how to I re-configure BM from using 2 NICs with
NAT to a single NIC
with one private IP address to proxy HTTP.

Hope this all makes sense to someone !