Sparking netural..

There is current returning through that neutral block from one of the other lights upstream of that fitting. Disconnect the neutrals, and one will be at 0v (near enough), and the other will be live, as there's no voltage drop across the 'on' light, because no current flows. I learned very early on that you can get quite a belt from a neutral that way!

Neon driver! Bin it! They are the work of the devil and about as reliable as a Politician. IMHO you have either isolated the wrong circuit or you may have a borrowed neutral. Either way...get help I feel that you may need it.

Ah, didn't notice the first bit that you had turned off the MCB for that circuit. Forget what I wrote above!

Crossed neutrals. I had the landing light connected to th downstairs live and the upstairs neutral. Discoverd this (loft lights were connected to landing light) when I was moving an upstairs light. Big flash and darkness in the loft when I disconnected the supposedly dead neutral. Wiring had been done by a well known building Co., nic/iec registered, so part Pee compliant???.

You will have a test certificate so report the company to the health & safety executive.

You're a frecking good example why we now have Part Pi(iss)...

A couple of possibilities...

As others have said there may be a crossed circuit of some description.

You did not say where you got the spark from (i.e. if it was to the neutral connector itself, or to earth). If it was to earth you need to remember that a neutral ought to be regarded as a live conductor. It may have a voltage on it that is some way away from earth potential. How far will depend on how "stiff" the supply to your property is and how many other properties share "your" neutral. The result is that it can be a few volts above ground - certainly enough to pass a reasonble current to earth.

So, using an 'authorised' company would have prevented that, eh? Just like the last poster's story....

Yup it was certainly enough to throw my glasshouse breaker when earth was touching neutral on an otherwise disconnected circuit.

Pete