Hi
Thats not an overflow, thats a pressure relief valve.
Thats an important safety device and would advise you get a corgi registered gas engineer in to deal with it.
You cant shut that off and working on any appliance if unregistered with corgi can get you a £1000 frine and 6 months in prison, not to mention risking the well being of you and your family.
Subenterprise
January 2007