Went on canyon support and a nice chap called nick said M4 x16 does the job. Screwfix sold me 25 for a couple quid, so you have to wonder why it's too much for wahoo to provide two!
My question is what they are up to with their cockpits. There don't seem to be any available - I could probably do with a slightly shorter one, but I don't see them on the site. They had a firesale of all their stock in March but now nada :(
Cheeky sods for starters