Thanks Warchild I will keep Barrow in mind when looking at fittings. So much choice, hard to choose.
It's looking more like I am going to go down the soft tubing route. I think I have decided on Barbs over compression fittings. I have watched Tom's Watercooling for beginners video and found it very informative. He recommends 7/16" ID Tubing with 1/2" Barbs as they will hold just as tight as what clips would do. I am assuming that this is still the case?
Yes barbs are very effective, but I have never been keen on them. They do require a cable zip tie to ensure indefinite tight fit making them quite ugly to look at. Compression fittings are the best option. Considering most of the world transition to hard tubing now, you may be able to get a good deal on them. I used barrow below as an example to show how much they look like an upbranded Bistpower fitting, but you choose which you find is best

Of course, if you dont care about aesthetics as much and want performance/efficiency, then there is nothing wrong with barbs.
As for tubing size, it really doesnt matter. Just remember, this for soft tubing. The narrower the guage, the more chance you have of the tube forming a kink. If you go for beefy thick tubing, if can limit flexibility if you have some tricky angles to train, but its rigid enough to prevent kinks.
My first tubing was 10/12mm ID/OD which was easy to setup but did have an annoying kink. I then moved towards a 19mm OD which was a good choice for me. The temp difference and flow between these are very little so no advantage is gained in going for thicker tubing really.
Watercooling is now all about personal preference.