after speaking to Jacksons tech support found out that the plastic extension piece is only supplied with kits 50mm and above and the plastic extension did the job perfectly for me as i had a 25mm gap to fill between heki and roof lining, actually did not need more metal clips but it comes as a kit unfortunately. Jacksons could make this more clear on the listing but if your reading this, now you know... :-)

Works well but makes a massive assumption that your ceiling thickness is only ever going to be 6cm max. I've had to buy 3 of these not because there's anything wrong with them but because our insulation was so thick. I've had to dremel the tops as they don't just brick together as you'd expect. Shame they didn't think about it at the time of design as it would have made so much more sense and have freed up 2 hours for me to do other things around the van. Quality service though from Jacksons.