Ceiling light wiring problems, please advise
Standard UK wiring.

3 x blacks: neutral loop in, neutral loop out, neutral to other light.

3 x reds: live loop in, live loop out, live to switch

back/red: black (with tippex) switched live from switch. red (with tippex) switched live to other light. 

Turn off the mains supply.

Using a test meter/continuity tester, the led live to switch and black switched live will be open circuit when the lightswitch is off, and short circuit when the lightswitch is on. 

http://wiki.diyfaq.org.uk/index.php/House_Wiring_for_Beginners#Loop-in_Wiring