Hi Gilly
If I am understanding your question correctly, you have unattached the all the
Rubber hoses which attach to the top of the sender unit and the circular Bung nut which fixes the sender to the tank? But the sender is still firmly fixed in the tank opening? If so it is just held by friction of the rubber gasket which seals it to the tank. I have had similar issues. As I recall I inserted my left thumb as far as I could into the large hole(filler) and at the same time used a large flat screwdriver tip CAREFULLY inserted between the plastic sender top and the tank itself. Use mostly the force of the left hand/thumb grip to pull the left side of sender away from the tank then very gently(so you dont bugger up the sender lip)use the screw driver flat head to pull right side away from tank. 1/8 of an inch at a time go back and forth left to right and slowly but surely it will come out. The last one I did I even used a little PB blaster sprayed into/around the perimeter of the sender/tank seal in hopes of it getting in there and loosening. If thats not the part you are stuck on let me know.
Matt