£400, as per the question.
This is therefore going to be about 2 hours work - yes, these people really do charge at a rate of £1500 per day.
In a rising market (across much of the country) you have a choice, really. Accept it as a reduction in the offer price of £400, or state to the vendor that you'd be happy to provide the necessary deed of variation but since it going to cost you £400, you will wish to increase the price at which you sell by £400 to compensate.
Its a variation of the old trick of the buyer making an offer and reducing it post-survey due to some spurious and obviously spotted minor imperfection in the property that the surveyor comments on.
If the property market is rising in your ares you could call bluff by suggesting that you don't consider it to be enough of a problem to prevent someone else wanting to buy your property if you re-marketed it. Much depends on the relative timing of ypour proposed purchase at the other end.