How can I remove a draft in Autosave

If I have a lengthy or complicated thread to post, I usually draft it in Notepad in case of PC problems.

I was recently mid-way through drafting a post when I was called away. I later came back, completed my draft and copied&pasted it into a new Reply. At that stage Autosave (which I think is a brilliant feature) popped up to remind me that my previous draft had been saved - I'd been replying online in the forum and not in Notepad.

I declined to use the autosaved draft and posted my lengthy reply.

Now, every time I try to post something here, up pops the same message "Do you want to use the AutoSaved post?" - but it's the very old one, from the completely different subject, as above. And it will pop up again when I hit Save.

How can I remove it, please?