Weird ipad only problem

Why does my iPad 2 connection to my wireless BT home hub seem to stop every evening?

It's fine during the day, so password etc must be fine
It's fine if I connect in the evening via my iPhone tethering, so the wireless hardware is ok all the time
When it doesn't work, all other devices on the Bt wireless network connect fine, so its not the BT

Any ideas anyone? Bizarre!