I have unlocked the Legend achievement, however i have not been given the helioskrill armour so can a moderater help me out?
Try going back into MCC and change a customization item. Your armor, your banner, your emblem, etc. Maybe also try playing a quick multiplayer game; something to help ensure that Halo Channel updates your MCC progress. Then, go back to Halo 5 and check your REQ collection for Helioskrill.
It also helps to make sure that your region/language is set to English, I think.