That makes it 3 strange "Polish" letters I've received in the post. This one is the most cryptic yet. The envelope once again spells my name wrong "Mr David Gallety" and has a UK stamp, but a Polish post-mark. The only other marking is a small cross on the back corner (pictured).
Inside is a blank post-it note.
If you've no idea what I'm talking about, info on the other prank-mail I've been getting is here: letter 1 and letter 2. The kinda-boring plot thickens!