Audi 200 fuel pump replace

[Modus]

So I'm trying to replace a failed fuel pump after the car stood for 5 years. The old pump came out without any plastic collar and without the filter sock, and I can't get what I think is the plastic fitting out of the tank. The central section is wobbly, and will lift say 15mm, but I can't get it out. If it wasn't for the filter is missing I'd fit the pump, but the tank is poor condition and needs that filter.

Am i right in thinking that centre section should come out and the new pump should fit into it ? If so, any ideas what is holding it in ? All the manuals I have make no reference to this, they just say take the pump out suggesting it shouldn't be a battle.