the "middle bit" is just a flat plastic infill piece, you need to remove that at that bottom joint then lift the seal off the window frame, and measure the thickness of the frame ,to get the size
stu
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets