Odd wiring in ceiling light fitting

I've removed a ceiling light fitting in the bathroom of my 'new' (built c.1960) house, and I'm a little confused by the wiring - perhaps someone can
suggest what is going on -
The ceiling light fitting has three terminals, of which one is not used and the other two have black wires running to them - one each. The two red wires are joined together and insulation taped. The two earths are also twisted together.
The light works fine and switches on and off with a pull-cord switch located elsewhere in the room.
The problem is, the light fitting I am replacing it with has a Live and Neutral side, and I don't know which wires to put in each side - I tried the two blacks, one in each side (as per the current light), nothing happens - then tried the two blacks in one side and the two reds in the other - the light comes on, but permanently on irrespective of the switch position.
Any suggestions gratefully received.
Cheers, D.
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Sorry, should have added - I am in the UK.
D.
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David Wright wrote:

OK, sounds like one of the reds is the incoming live - it is joined to the red of a twin&e going to the switch. The return from the switch is one black going to the lamp. It should have a bit of red sleeving on - but may easily not. The other black on the lamp is the neutral return.
Putting the two reds and two blacks together should have blown the fuse the moment the pull switch was pulled - the switch would short out the supply.
Is the new fitting significantly higher wattage than the old? If so, I would suspect that you have a loose connection in the pull switch. It was connecting well enough to pass the low current of the original lamp, but not making good enough connection to power the new lamp. And, of course, not good enough connection to pass the short circuiut current when mis-wired.
I would suggest reverting back to the original connections - then go to the pullswitch. If it obviously has a loose connection, tighten it. Otherwise short it out by putting both wires together in the same hole. Then put the power on. If the new light then works - you need a new pull switch.
--
Sue

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