All the reds join together - none of the reds go to the light.
One of the reds is power coming in
Another red is power going off to the next fitting
A third red is power going off to the switch
One of the blacks is neutral back to (from) the consumer unit
Another black is neutral running to the next fitting
These two blacks connect together and the new light connects to them as well
The third black wire (your challenge is to identify it) is not a neutral at all - it's switched live - live only when the switch is set to on. It is this wire that connects to the other side of the new light.
Sometimes it has an identifying marker sleeve on it to show that it's switched live.
The earth connects to the metalwork (if any) of the new fitting - for safety.
Remember Lord Finchley...
Lord Finchley tried to mend the Electric Light
Himself. It struck him dead: And serve him right!
It is the business of the wealthy man
To give employment to the artisan.