Note to anyone planning on drilling a tank themselves - regarding weir placement.
If I had known I would have a full height weir I probably would have drilled through the bottom of the tank, to minimise wall clearance required. Just a thought for others. In reality though, I'm happy with where they are because I'll be able to reach the taps where they are. As I have limited mobility, for me to access taps inside the cabinet I would have to lay flat on the floor & do it as best I could (I can't squat down).
I can get down on the floor and up again but I need something to support me and it's a somewhat painful process. So if anybody has restriction of movement, keep this in mind when you are planning.
Also after discussions with my mate who owns a very successful and large plumbing business, I decided Not to use a gate valve. Apparently although they are very good for controlling flow, they are not designed for it and will eventually fail. The gate tends to warp sideways unless they are kept fully open or fully closed as they are designed to be used.
Whether they would fail under home aquarium conditions of flow, pressure, etc, I don't know & nor did he but since I'm building this for the long term I decided not to risk it. Another factor was that I would have to use a pvc gate valve to avoid corrosion. PVC gate valves are hard to get & inherently weaker than their metal counterparts.