Strange web access behaviour with Dual wan router

I've got an Edimax BR-6524K dual-WAN router, with a 2Mb Zen internet
connection and 16Mb Sky Broadband connection configured. If I
configure the WAN policy at 50/50 between the two WAN connections I
get no problems, but don't get particularly good download speeds
(about 3Mbps).
If I set the WAN policy to 80% Sky and 20% Zen, to take advantage of
the additional Sky bandwidth, I find that I'm unable to access any
web pages from most (but not all) of the PCs on my network.
I have no QoS settings, and other settings are pretty much default.

Does anyone know why I get this loss of web connectivity when I shift
the WAN policy?