Spring in toilet flush

[Gecko20]

Hello!
A plumber came out to fix my toilet yesterday and replaced the fill valve, which works fine now. However, when I got home from work and wanted to check what he'd installed, the flush button had been jammed on quite tightly, so when I went to unscrew it and lift it off to take the cistern lid off, it took a bit of effort and then it when it did come off, a spring popped out and I don't know what part it came from!

I did try pushing it back into the bottom of the button mechanism which worked temporarily, but this morning the button got stuck down making the toilet flush continuously, so I took it back out. I've taken the button off many times before and don't remember seeing a spring in the bottom (unless it was tightly attached inside at the top), so I'm guessing that's not where it should be?!

Can anyone help? I really know very little about plumbing and I've spent ages looking online but haven't come up with anything.

 

[just pumps]

Does it not go over the highest section in photo #1 and inside the lowest section (the tube) in photo #2

 

[Gecko20]

I did try poking it into the tube but it didn't seem to stay in very well and then after a while the button didn't pop back up after flushing. I'm wondering if it was originally stuck higher up in the tube more securely which is why I haven't seen it before?