Nothing is for free.
You charge visitors for a cup of tea round your gaff?

Although the usual way this is done in my experience is that you are given a free DB instead of a £300 discount (great for the dealer and salesman) it is possible to get stuff like this for free if the salesman is willing to pay for it himself. A very obvious example would be if a family member were to buy a car and the salesman did not wish to make money from them, they could pay for extras up to the value of their commission themselves.
It is also possible for general Billys to get a salesman to do the same thing. For example you could visit/ring a dozen dealers and play them off against each other and eventually get down to the actual cost price of a new car for the dealer. So if you were to have rung me when I was selling cars and said I can get this car for £X which turns out to be the lowest I can sell it for, to get the deal I might have paid for a set of carpet mats or whatever myself.
DB is interesting in that the customer thinks it's value is £300ish and with a tame cleaner a salesman could probably get it put on for £10 cash in hand. It is even more likely for this sort of thing to happen if the sales staff are paid more per car for hitting certain targets. As a very simple example say if you sell 9 cars in a month you get paid £100 per car and if you sell 10 you get £150 per car you would probably be very happy to spend a few quid to sell car number 10.
Although I suppose it is theoretically possible to get a salesman to pay you some cash for buying a car from him I have never heard of this happening and it would not be looked at in a good light by management I'm sure so I believe that you are pretty much getting something for free doing this.
My experience of course was in a high volume dealership with brutal competition nearby. Mpower works in the rarefied atmosphere of the prestige car world and for him it is probably totally different.
Junior if DB is something that you feel has value for you why not offer the salesman £50 cash (off invoice) for it. I would have been delighted to do that.