I have bought some lyriks and wondering whether a 21cm steerer will fit a mega?

According to the Nukeproof site the head tube is 115mm – allowing 20mm for the headset, 20mm for spacers and another 50mm for the stem you should have no probs.

Depends on the steerer tube width (tapered, standard, 1.5?) as it will determine whether you will need an internal or external lower cup or even upper cup. What sort of steer tube on the forks (and what travel)? What’s your stem stack height?

1 1/8” steerer and 170mm suspension

You need one of these:
Nukeproof headset 44IISSDC
Keeps the front end low as you don’t want to slacken the head angle any more than standard. Very low stack height, should be fine with your steerer. Just checked, stack height is only 12mm. You’ll be fine!
PS: It’s the deep cup version, works great on the Mega.

cheers 🙂 ill get that