OP, To me it looks like a very subtle form of bullying to me. (ask your neighbours, if this happened to them aswell?)
Just write a note to the "addressee" to be attached to the dated & timed delivery note.
That you'll give him/her 6 hours to collect the parcels from you at your convenience (be out for 6h & 1 min), otherwise you will charge them 30£ per parcel & the money will go to a charity of your choice something like "The Homeless Elves of Iceland", remember to give a receipt for the money taken & a copy of the receipt that the charity e-mails acknowledging money recieved.
Or you just charge them a massive amount for storage & delivery...40£ per parcel, sounds good to me. 
Just to muddy the waters abit...
What about post addressed to the previous tennents, who now live a few doors down, for the past 7 years?
We did take their post to them. But in the last 3-4 years, we decided not to give them their post (especially the important stuff, like mobile phone bills et cetera) and just scribble "not known at this address, return to sender", then pop back in the post box.
My defence is...if they can't be bothered to inform the revilent companies that they've moved, then we aren't obliged to deliver their post.