A bit more information about my issue.
I actually use my broadband to connect via a Citrix session to a remote desktop and I also use a VPN on occasions; I was suffering with constant session dropouts, I've had 3 engineer visits with no cure, so I invested in the dual wan router to provide failover to the (slow but sure) BT adsl line.
The router decides when WAN1 has failed by making occasional ARP lookups like it was doing DNS work, if the lookup fails it switches to WAN2.
Now I have this kit in place I can see what was always going wrong but which I wasn't able to pin down, the dropouts are caused by the ARP/DNS services stopping for a few minutes (typically 3 or 4 mins) then recovering. If I were to reboot the cable modem during this time the ARP/DNS comes back instantly. During this dropout period all the actual ip traffic continues fine with no noticable disruption.
Does this scenario give any deeper insight of my issue, does it seem familiar to anyone? I'd really like to get this fixed as I'm teetering on the brink of cancelling my expensive 20Mb contract because the dropouts are so frustrating.