My bad about the parts confusion - typing too fast and clearly got mixed up!

The setup is like SJX426's first photo. The issue is that the two lugs at the back are pinched together to fit the smaller post. The seatpost itself seems to be alloy in the end.

I'll try and pry open the gap as you've both suggested. I've also read about sliding a coin in the gap and prying it open by pressing against it with the binder bolt - slow and steady in any case, and we'll see how it goes. I'll dig up a seatpost closer to the "right" size and see how it fits in the tube once the lugs are a bit more open. Will post an update - and hopefully will have enough posts to attach a photo by then!

Thanks all