Any idea why my Steamglide 2200 leaks water from the soleplate?

I bought a Russell Hobbs Steamglide 2200 steam generator iron a while ago without a manual. Occasionally it leaks large amouts of water from the sole plate, which understandably means it's useless to do the ironing with! Any ideas how to prevent this? I have no manual toi refer to so have no idea if it's something I'm doing wrong!

It's now go to the point where I'm on the brink or replacing the (virtually new) unit as it's just too frustrating to put up with!