Ceiling light permanently on - pops breaker when switched off?

[Taylortwocities]

This leaves two more blacks which are both neutrals

Not so. Just because the wire is a black colour does not mean it is a neutral!

Blooming 1960s houses and their old electrics…

Also not so. This is standard loop in wiring that is done today. The loop in can be done at the switch or, like yours, at the ceiling rose.
Your only problem has been a lack of understanding of how the wiring of lights is done.

 

[robinbanks]

Don't forget, it's also possible to have neutral wires show a "ghost" voltage that is induced because they run right next a wire that really does carry a voltage for their entire length. The changing electric field in the live core will generate a voltage detectable by a meter in the neutral core

People make mistake changing lights all the time usually because they pop the ceiling rose off and are faced with 3 reds/browns and 3 blacks/blues (unless it's the last one on the circuit). They also have a brown/blue from the modern light so all the colors get put together by colour. This puts the light permanently in circuit and allows the switch to short live to neutral popping the breaker. Always take a picture when you change electric fittings. If you're faced with multiple wires that look the same use a sharpie to put 1, 2, 3 etc dots on the grey sheath before you take a picture

The usual mistake:

Wire from the last light comes in on the left, wire to the next light goes out on the right. Putting all the lives into one and all the neutrals into another causes a problem.

From your description you seem to appreciate the switching but if you wire this:

There will be enough of a live coming back from other downstream lights switched on to illuminate the local bulb, particularly an led. Flipping the switch shorts the live and neutral and pops the breaker.


One of the ways to wire it working is like this:

*(ps; I've talked about it like it's DC because it's easier to understand)