removing wooden sash replacing with non sash upvc d/g

[scoobydoowantsu]

Hi,
I have used this site lots in the past, but always found what i needed till now.
I already fitted one window this way a year ago, but now im doing 15 windows, the rest of the house.
I enquired before fitting the last window that you could simply take out the sliding part of the sash window and fix the double glazed frame to the inside of the space that was left.
it worked well, but i just wanted to ask if anyone has done this before, and as im doing 15 windows, if anyone had any tips and ideas for me?
I have already checked with the council as regards to the law, and the requirements for building regs.
So its just fitting now.
And, does anybody know the average mm2 of a trickle vent?
Thanks guys, and great work here.
simon