I had a problem with mine (a 406SV ) about four years ago soon after I
got it. I forget precisely what happened but I was able to fix it by
gentle surgery on the plastic. In my case, I removed the top (of three)
black flap which is part of the fuel lid assembly and which, with the
other two, consitute the 'spring'  which makes the flap want to open.

My recollection is that the lid itself simply pulls off/clicks onto
the hinge pin - I think, in fact, that my problem was some sort of
breakage on the black part which caused it to lever the lid off when
you attempted to close it. As I say, there are three black 'springs'
and the remaining two do the job OK.

I think the essence of Nigel's and my experience is that if you
carefully work our how the lid open/closure works, you may be able to
solve the problem fairly easily - surgery rather than replacement.

I also had a problem with the catch sticking - that is, when you opened
the lid by pulling the trigger to retract the black catch, it stayed
retracted and the lid would not close.

Again I worked out a fix. You could get it to open by tapping around it
and/or by flicking the trigger. I think I sanded the catch a bit,
tidied up around it and applied lashings of graphite powder - the stuff
you use for locks.

Also I adopted the habit, when opening the flap, If letting the trigger
go so it snapped back, rather than lowering it as one tends to do.

Well that problem has not recurred. In both cases the fix has worked
for years (touch wood)