Parcel Force are c**ts

Oh god this thread has brought me out in a cold sweat. Poor you DixonB. I have nothing but sympathy.

Have you thought about contacting the police with regards to your neighbours? Surely, if someone has accepted your parcel, this is theft?

I think that if you are sent something by mistake ie theres two of something in the parcel instead of the one that you ordered/paid for or theres a lens in with your dslr that you never ordered/paid for then its the company-that-sent-you-it's responsibility to realise, inform you and have it collected. You have no obligation to own up to having received something that you weren't meant to.
Probably a similar situation here. Unless the delivery driver asked the person "are you joe bloggs and is this flat number 1a" and the person lied and said yes thaen i can't see that they would be guilty of anything.
Anyway, there'd never be enough evidence to convict someone of theft.
It does suck though that someone would do this.
Look out for it being moved. If one of you neighbours did get it then i doubt they would be stupid enough to build it and ride it themselves but they might sell it on ebay/get a junky mate to come round n take it away in exchange for sum crystal meth.