I would welcome any advice about replacing an old window with a new PVC one which is not quite the right size, please. I have, after enquiring, bought the 'elargisseurs' from a popular bricolage shop and they are driving me mad. When trying to clip them on to the side/bottom of the new window they commence correctly but as I progress along the length they unclip at the beginning! I have also tried gently tapping them in with a rubber hammer, with the same result. Also tried sliding them in with no luck. I am now considering screws as a last resort! As a final insult they clip to each other beautifully so I am now wondering whether it is the window which is at fault.
I have checked this out with a French neighbour who is well versed in renovation. He tried all of the above with similar results and also said he had no idea!