A trick I picked up from an old HJ column in the Telegraph is this.

Put a cupful of vinegar (doesn't matter if it's malt, spirit, white wine or whatever) in a shallow bowl, like a breakfast cereal bowl, put it in the car in the footwell next to the one that had the milk spillage. Close windows, close airvents, close the doors.

Leave overnight, or longer if you can. Remember to remove the bowl before driving off :-D

The very worst substance, you would have had to remove the carpets and underlay at the time and thoroughly wash every single drop of fresh milk off still wet to avoid this.

Once dried in you're in trouble, you have no option now but to remove the carpets and underlay, then see how badly the smell has leached into any fittings or rubber or plastic padding under the carpets.

You might get the smell out of the floor, but you have to fully wash, soak for hours, the carpets and underlay, and allow them to dry for several days, it might work it might not, it might be prudent to start looking for replacement carpets from a breakers, seriously.

An ex colleague of questionable use (not for this reason) spilt a cup of coffee in my lorry, he cleaned most of the spillage up but a spoonfull of coffee (so a tiny drop of mil content) seeped under a rubber pad and dried there, several months later the stench was overpowering, i traced the smell and found this 2" round patch of ingrained stain on the underside of the rubber pad, the smell from that was astounding.

Steam cleaned it, bleached it, ran it through the dishwasher, you name it, never did get rid of the smell, indeed after all that a direct sniff could knock you backwards still, had to throw the pad/liner away eventually.