cactus

hi could anyone tell me what sort of cacti this is? and should it turn purple?
i was given it around 2 month ago, off a friend who has had it 35 years! i am worried that it may be dieing. its around 45inches tall from the top of the pot!! and it is located in the conservatory now! not in the bathroom window as in photo!

regards
paul

 

More on the order of Euphorbia trigona than a cactus.

 

since moving into conservatory some of the small and new stems are turning deep red/purple?
does anyone know why? will it be the sun as its in a very sunny spot?

 

Might also be a monstrose form of Opuntia monacantha.

 

ive added a few more pics for u to look at! the old fella that gave it to me bought it in 1978 this cactus, never forgive myself if it died on me!!!!!!!