Why does my iPad keep disconnecting itself from my Wi-Fi

A couple of weeks ago I noticed my iPad had disconnected itself from Wi-Fi, so reconnected and thought nothing of it. The next time I turned it on, it had disconnected again (Wi-Fi was still on though) so I clicked in to settings again and back onto my network. All works fine - good - but then the NEXT time I turned it on (from sleep, not fully off) it had disconnected AGAIN.

This has now become the norm for my iPad. Constantly disconnecting itself from the network and doesn't reconnect automatically when I fire it back up.

Can anyone think what might be causing this, it's pretty annoying.

(I've tried switching it off fully and back on. No dice...)