ok, so its the Filler unit thats bust, thats a complete (big'ish) part.
if you can push it in then it shouldnt be a problem for the time being, other than the fact you may get petrol surges up into the filler unit that might occasionaly leak out the cap, though it shouldnt unless you hit corners hard with a full tank of petrol.
i would suggest you refrain from filling to the brim for the time being.
if my laggy1 dont sell in the next few weeks i could rip mine off when i break it for parts at the end of the month.
when it is time to change yours, make sure you have less than a quarter of a tank of fuel in it, as it will make changing easier (and less risky).
to change the part, remove wheel, remove inner wheel arch plastic trim, undo screw in filler neck, release jubilee clips around fuel tank neck hose at bottom.
undo the x2 bolts on the inner wing that hold the filler unit in place. release the bottom hose so it is free of the main tank, lower the filler unit down, try and see if the rubber breather hose (that connects the top of the main tank to the upper neck of the filler unit)can be released, it should come off(has compression or jubilee clip).
remove filler unit, make sure you have top rubber washer fitted to new filler unit, reinsert into arch follow removeal procedure in reverse.
replacement part from a salvage yard probably about £30 or so, new your looking at £150+.