What is a good finish coating for my timber, it will need to protect it between erection and covering.I would possibly like to darken the timbers a little as ash is quite lite.

I've used Landark on ash, and it's nice, but it yellows up a bit. Any oil based finish is going to do that though. You can tint Landark. You can probably find Watco locally, it's not as protective, but you can get it in lots of pre-mixed colors. I like the color of ash as it is though.

Sansin has a bunch of products you can try. We've used their Classic before but found it stays tacky and is a finger/hand print magnet. They also have a Sansin product called Foundation that we have recently received. No real timber tests yet but sample tests seem promising, no tackiness. Claims to be a UV protectant.