Thanks guys, In summary it is not class legal to use a continuous 8mm line (bit of a daft rule but rules is rules) so this answers the question of whether an 8mm line would function in the mainsheet cleat. So its tie or whip them together.
Will look at this after I've changed the mainsheet as I have the thick and bulky Dart original and am looking at changing anyhow.
Are there also limitations on the sizes of other lines and lengths some form of reference table would be really useful.ie.jib
While I'm on the topic what is the approved new layout for the downhaul?
Graham Collett
Sprint 15 1941