Also known as the 'Little Polish Attack' because Bent Larsen was not a small guy from Poland.

Bent Larsen was Danish. I would guess it is also called the 'Little Polish Attack' because 1. b2-b4 is sometimes called the Polish Opening. en.wikipedia.org

I used to sneak in some Polish (Sokolsky) opening games back when I was more 1000-1200 strength OTB. Had some nice games with those unprepared against it. All I was trying to do was get people from playing the Sicilian so much. Like every game I saw back then was a Sicilian game and d4 games ended up closed. I wanted to get my opponent out of his or her comfort zone as much as humanly possible and impose my type of game. Usually this worked (Until my opponents ended up being 500-600 points higher with much more theory on my opening repertoire  )

I once had a theoretical discussion about the virtues of 1.b4 versus 1.b3 Steve a local master played 1.b4 twice against me in tournament play and I (an expert) played 1.b3 against him. Amazing I won a pawn in all 4 games, but Steve defended well and all four were drawn after interesting play.

This used to be my favourite response.

still is the main line

It is indeed the main line.

In some instances, ...f6 is OK in this line.

Can anyone advise me why black would take here, I cannot see an advantage in doing so?

Maybe because if black doesn't take white will, breaking up black's pawns?

White retakes with pawn implies a slight disruption to his structure; retakes with the bishop, the bishop gets hit. Otherwise, the WN on d4 is a good piece. Judgement call, but I might well have played it myself

Black could have considered building a big centre with ...d5.

Yes I think black should have played ...d5 at some stage.

I discovered the hard way that a move like c4 in this sort of position risks a wedge being driven in to White's game with a blockade at c3 (I had the equivalent position with colours reversed, so it was d6 that was the problematic square).

my reasoning here is simply that none of black's potential replies bother me. I like the positions I will get.

I personally been burned by c4 positional problems later on. So either I play it later or not at all.

my reason here was to make Mr. Songy think I was castling k-side, but not to actually castle k-side. I was looking q-side already.

I think it is going a bit too far to describe this as the losing move.

probably the losing move, missing what is to come. let me rephrase, this is an indicatrion that black is heading down the wrong road.

Losing move is a stretch, To say black has no plan besides development, I'd say that's accurate. I'm looking at both sides, and I'm asking what is black planning to do to gain the initiative here? How can he gain the advantage? The bishops (and queen) are immobile for the most part, and neither rook is able to do anything. Yes the knight is able to move, but then the e4 pawn has no protection. Not unfixable problems, but black needs to start taking an active approach to the game.

Worth considering were ...d6 and ...Bd6.