Does anyone know where to buy high quality Dynema up to 6mm. I've found some in marine supply places but they are lesser quality chinese made and have a more open braid which is possibly subject to bunching.

Kite shops should have Dyneema off the roll. If you get desperate

For those that don't know - Marlow Formuline's point of difference is that it is a 12 strand Dyneema(UHMWPE) otherwise known as Spectra. The 12 strands are in a single braid as opposed to having a separate jacket, which are usually a cheaper HT Polyester and not as abrasion resistant. The cheaper stuff is fine for outhauls but when it wears it can bind.

For use on Windsurf downhauls in particular slalom/race a solid dyneema braid will last longer and maintain its circular shape better - I'm a bit od a tight arse so I rotate my downhaul out to my outhaul when it gets tired. I also leave it full length on the boom and carry a knife in case the boom clamp rope breaks and I need to Jury rig in no-mans-land.