Jake, how dissapointing for you! If, as you say, the green-leaved branches are coming from both above and below the nice red-leaved branch/es, then it doesn't sound like a grafted tree, although it certainly looks like one.

I've never encountered a purple-leaved Acer "reverting" to green, other than when grafted and, as Chris said, where the rootstock has taken over.

I would suggest that you do both: chop out the green stuff, it's not particularly pretty - but without it, you might find that what is left is not attractive enough to remain on display in the pot, so you could buy another one and start again.

Once you've chopped this one, you could put it in a plastic plant-pot and "plunge" it into the soil somewhere in the garden, This would keep the roots cool and moist, but you could easily heave it out again and re-pot it next year, if it regains it's form.

Mike, yours looks lovely, and no, it doesn't particularly look grafted.

I have a lot of similar-leaved acers, grown from tiny seedlings so they are definitely not grafted.

Hope this helps,

Rachel