I am replacing my old wood windows with upvc so have to take some accurate measurements. the existing windows have green plastic cavity trays which are tacked to the back of the windows. Im not sure which is the best way to go about this, weather to try and cut down the side of the window beetween the tray and back of the window to remove the tacks, or to horizontally cut sections out of the jambs and prize the timbers off the trays in the hope of leaving them intact, and fitting the new windows back into the trays. Or removing part of the tray and fitting the window back to the brickwork. The trays extend approx 75/100mm into the inner reveals. Any suggestions or tips would be greatly appreciated. Regards Chris.