Are you smoking Shack? Of course the bystander has a defence! Blimey...
OK, for the hard of understanding....for bystander read A.N. Other who handles the binbag, otherwise how would he/she know it was a firearm? Police arrive and responsible citizen (bystander) says "there you go mate, have a shotgun"
If it was in your garden but found by a third party then you are in
possession, rather like the OP. The onus would then be on you to prove that you had no knowledge of the weapon, had never handled it, and were not in possession.
PN.....even better. Ned the crim has just robbed the local Post Office and as he legs it through the estate side streets, casually drops his sawn-off in your garden. It is not found for some time. The onus is now on you to prove you were not in posession but it's on your property.
It has been emphasised to death that this is an absolute offence ie NO mitigation, NO defence.