Appreciate the advise guy's will spend some time going through the links provided.

It was the Karitek skeg that was 20cm so wont fit a Petrel. They are able to put the compression chamber inside the skeg box which will reduce the height to 18cm, still doesn't really help. Kajaksport do a skeg which is only 12cm but I have emailed them to clarify and get some prices. As you mentioned I should have some marine ply left over from the cockpit recess to build one so that is still an option. Kajaksport also provide more out-fittings for you boats including seats, knee supports, footrests, hatches and rims and little day hatches which fit under your deck. Thanks.

http://www.kajaksport.com/?components_eng/components.php&tuote=&ryhma=S%C3%A4%C3%A4t%C3%B6ev%C3%A4+ja+per%C3%A4sin