You appear to have a genuine emergency so therefore under the legality known as "self help" you are entitled to gain access and make the repair. To be on the safe side, serve written notice upon the estate agent, maintainance company and the occupant of the flat that unless within 24 hours the nuisance is abated you will without further notice gain entry and abate the nuisance yourself. Engage a locksmith to gain entry, let the plumber in to do the repair, leave all safe and tidy. Send the bill to the estate agent, including cost of repairs to your flat. Request payment within 21 days, if no cheque by then issue summons in the Small Claims Court.