the edges were not cut very well, they were quite jagged and so it was not possible to get a snug fit. But overall once installed, the products looks good and functions well. Not bad for a 30 year-old window frame!
Purchased a replacement blind and on removal of faulty blind noticed that the problem was a broken cord guide on one side,I purchased replacement cord guides from loft solutions not realising that they were Velux I now have a spare Velux blind.