Red and black cable could be as new as 13 years ago. The life span of such cable is well in excess of 50 years so no need to replace.
Perhaps the previous fitting overheated the end of the cable, in which case it needed cutting back a bit. Or perhaps the spark was looking for unnecessary work. Modern cable, whatever the colours, has a life in excess of 50 years.
I'm sure you may well be correct. The house is now 43 years old so it may well be original and coming to the end of it's life.
For me, I would feel a lot happier just getting it out of the way now and then I will know I don't have to worry about it again. Knowing there are question marks over how old and how much the install has been hacked about over the years will play on my mind so I'll just crack on and get it done.
Really appreciate the honest advice you have given here. Thanks