I've got to agree, £400, especially for a DE where you will see hardly any noticeable improvements is a lot of money.
I wonder if they would do a group booking discount of some kind? If tyhe MAP takes less than an hour, including loading it onto the rollers and setting up I would assume they have different maps stored and ready to load. That's also £400 after you've spent a few hundred doing supporting mods like the intake spacer, etc.