looking on the bright side, since it is a lighting radial, it would be reasonably easy to halve it, to identify about where the fault is.
A £10 multimeter, a step ladder, and a notebook are the essential tools. And some red sleeving. And some 6A chocblock.
I recommend also getting a fine permanent marking pen and some white vinyl tape to mark your cables after you have identified them.
If you have a neon screwdriver in your toolbox, go and get it, and put it in the bin.
Have a look at your lighting layout (remove any loose floorboard) and decide where the cable run is likely to go between them (it will probably start at the lamp nearest where the cable comes up from the consumer unit).
TURN OFF THE MAIN SWITCH and test for dead
open the ceiling rose of the lamp that you think is about half way round, identify and mark every wire that you see and draw a diagram of how they were connected so that you can later replace them exactly as found, then disconnect them and put each into a separate connector in the chocBlock.
You have now effectively cut the circuit at its mid-point.
Restore power and see if the fault is still present.
If so, it is in the upstream half of the circuit.
If not, it is in the downstream half.
Reconnect that rose, test that all lamps work again, repeat the test at a rose about half-way through the faulty section.
Do this a few times, and you will have a very good idea of where the fault is.
Look for signs of cable damage, water leaks, incorrect things connected to it, damaged light fittings, bad connections. If you have any junction boxes, be very suspicious of them. If your loft gives easy access to the wiring, inspect that first as it may be easy.
You could also test the resistances on the various sections, but someone else can write that.
