Older leaves tend to die back after awhile.

That said, your picture suggests there is leaf speckling, which can occur with spider mite infestation (look for tiny gray-white moving critters on the underside of leaves, or fine webbing).

If there's infestation you can cut back foliage and wash off what remains with a strong spray of water, keeping it up regularly until they're gone.

Colocasias in my experience don't do well indoors - full sun, lots of water and fertilizer is what they prefer.

Thanks so much for the advice, now you mention it yeah I had noticed in the last few weeks there was a web and tiny mites before I cut the leaf off. I've just checked the rest of the plant and same thing, webbing on the underside of 2 leaves. So I wiped them off, sprayed with water and it's now outside. Thanks again