I got this in a bag of pre-decimals, it is between the size of a penny and a half penny. It is not all that flat ether (seems to be little warped).

Is it coin or just a chunk of metal?

"Is it coin or just a chunk of metal?"

The latter. However this was possibly a coin after it went bad and rotted away, this mystery of what it was could go on forever..
Maybe one of our eagle-eyed scouts will identify the date and type of this coin for us

It just seems odd, as it doesn't seem to be the same size as any other of my pre-decimal coins. Which leads me to wonder what on earth it was doing in a pack of pre-decimals

Pre-decimals go back as far as you like don't they?
It could be this is a much older coin from say George III era.. or was a coin...

Hello,
You may want to put it on your roof for about a year and let the weather clean it for you. It might give up some secret then. On the other hand it could just be a plug but since it is a plug currently you don't have much to loose.