Link-local address being used by dns forwarder

Hi all,

I am running v1.8.5 on an EdgeRouter Lite.

I have configured a conditional forwarder to use a DNS server on the other end of my ipsec site to site VPN.

set service dns forwarding options "server=/myco.co.uk/10.1.5.20"

When I look at an interface capture for vti0 port 53 I can see a link local address is being used by the router when I  ping a server from my local network:

21:02:39.124970 IP 169.254.21.154.33184 > 10.1.5.20.53: 39205+ A? test.myco.co.uk. (39)

How do I configure the vti interface to use the router's IP rather than link-local?  I have tried setting the vti interface address but this just brings down the VPN tunnel. 

Thanks in advance

Richard