The sash windows (the ones that slide up and down) do not stay up. You prop them up when you need some air. (See photo.)
There is a twisted rod inside each side of the window around which a mechanism turns and holds the window in the position that you put it to. The rods used by the developers Galliard appear to have been of inferior quality, which is why so many windows have failed.
What you can do
Replacing these rods is not a do-it-yourself job. Call in a professional – a carpenter or an experienced ‘odd-job’ person may be able to do it. Steve Chalk, one of the Recommended Tradespeople has replaced rods for us.
Ask for a quote for the price first. Consider replacing several at the same time.