The problem is that with the information given I wouldn't like to commit to it just being half a day, could be a right faff, might have to pick keys up, no parking nearyby, possibility of a key meter and the hassle of trying to get the power on, shouldn't Imagine there will be a lot wrong, a load of C3s for combustible board, no RCD to concealed cabling/lighting cirucits/bathroom circuits (possibly even power if its not ground floor and it was done to a budget - but with 12 cirucits thats unlikely, this is a well spec'ed job for its time). I think you'd have to play it safe by going in at 8 times the hourly rate, surrey is much closer to London than the east mids, so hourly rates are probably pushing £40 down there, so take 8x38 = £304 and expect to loose it to the chancers who will do it it for £60 without even getting out the van.
If you were testing the whole block you could probably get through 2 a day, and you could offer them quite a substantial saving, but for a one off and too many unknowns could end up with a loss if you cut it too fine