Hi,this would cause me to wonder,if they did adequate testing,prior to the fitting of the new DB,and,afterwards.
A few days later,is not the time this issue should have come to light.
Reckon you are spot on there PEG they must have had a tripping RCD problem, typical borrowed N on the two lighting circuits= no initial testing, connect up, conduct bang test etc.
Are the lighting circuits fed from one or two MCB's?
Where did the feeds originate from,for the outside lights?
Have the MCB's been removed and inspected,or just tightened?