15 amp fuse in fuse box

Hi In old systems like that sometimes people interchange the fuses and fix 15 A fuse that protect Inmersor heater to lighting circuit that is 6A so that is wrong change as you said is only for three bulbs downstairs. I recommend you replace the existing DB fuses by Dual RCD DB automatic disconnection 17th edition that give you and the building a full protection. Jack Arrow Electrics Ltd

It's most likely that whoever last replaced the fuse did not have the correct 5amp rated one? or it could be that the 5amp fuse was blowing fairly often so someone has fitted a 15amp fuse to stop it blowing. Whatever the reason it's very unsafe and is a potential fire risk as lighting cable is not rated at 15amps and will melt and catch fire before a 15amp fuse operates. It is strongly recommended that you get a full Electrical Installation Condition Report carried out on the wiring in your home as there could well be other hidden defects.