Tethering via Bluetooth and ignoring the WiFi connection

I've just purchased an AirStash.

This means that the WiFi connection of my Ipad (3) is only accessing the local airstash fileserver.

I usually BT tether my iPad to my iPhone when I'm out. However, it seems that as soon as the iPad is connected to a Wifi, it wants to use this connection rather than the BT link for internet access.

The icon next to iPad in the top row changes from the wifi symbol to the OO linked icon when I connect to BT, but I loose my internet connection.

I'm there for assuming that the iPad is trying to get online via WiFi, not BT.

Is this likely? If so can I encourage the iPad to get internet access via the BT tether?

thanks