@opps fair enough. Thanks for the insight.
I cannot see how 14 sliding sash windows can be painted to a reasonable standard in a week. That is less than half a day per window.
Off the top of my head- assuming there is no significant wood rot and the windows are victorian (ie better quality pine) and that the parting beads do not have brushes, I would guestimate 1.5 to 2 days per window. That includes initial sanding, filling as required, sanding of the filler, spot priming, "caulking", 2 undercoats and two coats of gloss. Materials? Assuming Dulux Trade Weathershield- £20 for the primer, £45 for 2.5L of undercoat, £45 for the gloss, £40-50 for other materials and sundries. Total= about £160.
If your decorator doesn't need to sand through any of the paint then one coat of undercoat will/may suffice. I tend to use two coats of oil based gloss, primarily because I prefer the way it looks and hope that it increases longevity.
I don't want to malign the fellow and am mindful of the fact that I haven't seen the job, but I am concerned that the time allotted might meant that he simply plans to rub a bit of sandpaper over the existing finish and then apply one undercoat and one gloss.
Although he has been recommended, I would suggest that it might be prudent to look at an exterior that he painted a couple of years ago.
Sorry, I am not trying to coerce you into paying more than you need to, I just don't want to see a fellow forum member having to pay to have work redone a couple of years later.
In conclusion, I haven't seen the job so I can only limited advice.
Best of luck.