Leaking
Does the leak appear to be coming from the boiler?
Can the customer see / check any pipe work above the boiler to make sure water is not leaking onto boiler and running down. ?
Leaks from Condensate pipe should be referred back to installer.
If leak described as outside from the flue, this may be a result of pluming that can sometimes drip from flue.
If boiler is fitted on a sealed system and continually losing pressure, check whether water is leaking from the copper pipe outside the property, Check system pressure is not above 2.5bar, if yes check filling loop is off / disconnected, then reduce pressure in system by draining a small amount of water out of the system, the installer may need to do this.
If it is a new installation installed within 1 week customer should contact installer to check and advise if actual leak coming from the boiler