....but you're not telling me that that they're not still making a healthy skin on 300, of course they....it just makes you wonder that's all....
Not necessarily. For mass chain merchants - no matter what they sell - you'd be surprised at what maths the pull which makes "logical" sense. It simply doesn't follow that they'll definitely be making any profit on that £300 bike, let alone a "healthy skin".
Because of economies of scale, its quite normal for large retailers to sell selected lines at cost or even negative margin. As has been said, it could be old stock that they require to shift, an incentivised discount from the supplier or intended loss-leader.
Even though they may normally sell at £600, the "normal" cost may be (for example) £400. I haven't a clue as to specific prices, but just for arguments sake. The retailer may go to the supplier (or visa-versa) and say we want to stock your product as a sale item, give us 5000 at £200 so we can sell them at half price. In return, we'll give you prime product placement in our stores and advertise on TV. The benefits to both parties are beneficial here as it gets customers into the retailer and gets the supplier's bike advertised and brand awareness up. There is of course always the potential to get punters switched to a more profitable model once they are in the door. There's still profit in it, just not as much for the retailer or the supplier in the interests of stimulating business.
Similarly, the retailer may simply take it upon itself to reduce a bike to cost (or even slightly below) without supplier involvement. When there is a big market spike (as cycling will experience over the summer months/TdF) competition for this extra custom is a bit cutthroat. If you have one bike that you make no money on, but by advertising it heavily it gets people through
your doors where you then have a chance to up-sale or push profit rich add-ons to, then that is perfect retail sense. And customers who shop in your store once are more likely to come back and shop there again in the future.
I spent a decade working for an electrical retailer (not the one which went belly up last Christmas), and you saw this kind of thing every week of the year. The big boys can afford to pull this kind of lose-on-one-ticket to gain overall tactic because of volume. You'd seldom see a LBS or independent pulling the same kind of tactic (outside of discounted clearance or end of line) because they don't have the same flexibility or market potential.