thanks for the input everyone. keep it coming, as i said this is a prototype, and already i have several ideas for changes.
the nut i am planning on making will be from rather hefty brass stock, which will lock the strings (i hope!!) so yes, once string is in place, i will pull as tight as possible with pliers, and lock it.
my main concern is the travel for tuning as well, i would like to have at least 30mm, something i need to work on.
i am most likely going to remove the springs, i don't think they will do much in the scheme of things.
i will eventually have the tailpiece cast in brass once my mate is set up to cast it. at the moment i can get it cast in aluminium for free, so beggars can't be choosers! unless i can convince my wife that i NEED a milling machine
cheers
darren