Thanks! Yeah, I've read in various places that a self-builder should be able to put together specs for the various works packages and put them out to tender, but I'm really not confident, which is why I think I need a QS.
We've made some of the decisions already, including the timber frame, so hopefully that should help any QS we appoint.
By the way, if the superstructure is usually 15-20% of the build cost, is there a standard breakdown floating around somewhere showing the proportions for each stage (groundworks/substructure through to finishing the house internally)?