It may need the hinges replacing. Fire escape type windows which are large and open outwards at the bottom are particularly prone to this over time. The hinges take quite a lot of strain and the weight of the window pulls on them and then the fittings round the edge which lock into the frame. You end up with a poorly fitting window.
if they are replacement windows, the frames may be a poor fit in the walls. There will be a plastic trim on the inside of the house hiding the gap. See if you can prise it off. If there is a gap you can fill it with expanding foam (instructions are available, it is very messy) but the outside should be sealed with silicone to keep the rain out.
If you get some joss sticks you will be able to see if air is blowing in or out.
there will be a local specialist who repairs plastic doors and windows. There is a steady trade in this work in every town. It is a small business, not a Replacement Windows Company with glossy salesmen and brochures that you need. They probably also repair misted units, which is surprisingly cheap.
If you ask around, you may get a recommendation. If searching, avoid interweb recommendation sites and anyone with a non-local phone no. (it will be a clearing agency) or who does not show their local landline number and a real local address.
If you start with having them repair one window (should be less than £50) you can form an opinion about their charges and workmanship before you decide about having more done.