Where this wire meant to go?

its at the back of the alternator and causing my battery light to come on

 

Off to the alternator light on the dash. You might to re-do that connector.

 

So the wire it connects to should be round about there then ?

 

Yep, should come from the same loom as the thick wire to B+ on the alternator.

 

Don't cut the red wire!!

 

Its meant to be from the the blue wire with the chocolate block hanging down.
TBH, the bin is the right answer or over-the fence.
Fit a spade on the blue wire, does that wander off to a split charge relay coil wire by any chance ?
Put an end on which has a male and female spade, put the blue wire onto the tag at the front of the alternator( towards front of car) , put a female spade on the split charge relay wire and stuff that on the male part of the blue wire connnector. And do it with a proper ratchet crimp tool not one of those flat and painful pliers type abortions provided by all the usual suspects.
On ebay from £8 upwards.