Originally Posted by echinosum

You are posting on a UK gardening forum, so I assume you are in UK. Chaya is a tender Mexican vegetable that is not grown in the UK and is too tender to self-seed as a weed here. Also it has very obvious white leaf ribs on the upper surface of the leaf. See picture of leaf further down the page http://en.wikipedia.org/wiki/Chaya_(plant)

As the previous material I quoted indicates, giant hogweed contact results in photosensitivity, so it will get better over night, 'cos its dark, and then worse again as you expose the contacted area to sunlight again. Keep the affected area covered with clothing for a week or so for relief.