Identify the sender of my mystery post

I got some post today. Nothing particularly unusual about that fact. What is out of the ordinary, though, is that one of the items addressed to me was an empty envelope. No indication of who it's from. No return address, no proper (stamped-on) postmark, and obviously enough no contents. There are 3 things which could, I guess, lead in some way to a guess at identifying the poster.

Exhibit A: a number above my name. What could it mean? 132 has no signifiance to me.

Exhibit B: a Royal Mail postage paid "stamp" -- on the sticker which also had my address and the 132. What does SWDO 754 mean? Who uses stickers like this?Exhibit C: a not-quite-barcode thingummy. Means fuck all to me.

Beyond the normal cycle of bills and magazine subscriptions, the only post I'm expecting is a couple of replacement bank statements from HSBC. But they aren't due to arrive for another couple of days, should (I expect) be in an envelope with HSBC's return address on -- like everything else they send out -- and would probably not be addressed solely to Mr Darren Foreman.

So. Er. Anyone?

EDITED TO ADD after a request, here are two more photos.

Exhibit D: the full envelope. It's A4(?). That's a pen on it so you get the size and, as requested, "that 'look'". Hi Chris.
Exhibit E: nothing.