Since it appears you only have a double socket permanently wired into the g
arage, then if you want to make the lighting circuit into a permanent and p
roper installation, then I would do the following;
Switch off power to garage
Fit a Fused Spur Unit (FSU) next to the socket and using a short length of
T&E cable connect the SUPPLY terminals to the L, N & E in the socket. If yo
ur sockets are near the door you can get a switched FSU and you will have n
o need for a further switch. Make sure you sleeve any earth wires with gree
Run 1mm2 T&E from the LOAD terminals of the FSU to your switch if needs be
and from there to your light fitting.
Wire the switch as in your photo remember to sleeve the Earths and connect
Wire your light fitting again sleeving the Earth ensure you have correctly
wired the L, N & E.
Replace the 13A fuse in the FSU with a 3A.
You should now be able to switch the power back on and your lighting circui
t should work leaving both sockets free.
You say that your current switch is not working even though it appears corr
ectly connected L & N wise. I would check you have L, N & E connected prope
rly at both the socket end and light fitting. If you think it is the switch
still faulty try swapping the wire in 1way to 2 way. Internally the switch
is just a changeover switch that flicks from one terminal to the other. Th
e only reason they are marked thus is to ensure when the switch plate is co
rrectly orientated on its back box it is a downward press of the rocker tha
t performs the switching. If it still does not work return it to B&Q, aitho
ugh i might be tempted to test it with a multimeter or a low voltage lamp a
nd battery first.