Opal Glass Twin Lamp Wall Light

You state that this is "up to 40W". Does that mean 40W combined or 40W for each bulb i.e. 80W for the whole fitting?
It is maximum of 40 watts per lamp holder. So 80 watts would be the maximum for the whole fitting.

If I mounted these lights at 1700mm off the floor, would I see the lamps if I were sitting down in an armchair - or is there a diffuser on the underside?
The fitting does not have a diffuser on the underside so you would be able to see the 2 lamps inside the fitting. You could use opal lamps to reduce glare if you wish.

I would like to know if the light can be mounted vertically as well as horizontal?
Great idea, yes this wall light can be used either way up. You would see more of the lamp when vertical but just use opal lamps to reduce glare.

How near to the ceiling could you mount this light? Would two of these lights be sufficient to light a room (c 10ft x 12ft)? Could you use low energy bulbs instead?
You can mount this fitting as close to the ceiling as you like. They are primarily intended as a feature light and would not throw light far enough in the room to illuminate it on their own. The way the lamps are situated does not allow enough room to use a low energy lamp as they are typically longer.

Can I check how these are mounted to the wall please?
There is a shallow rectangular mounting box containing the wiring block. This is mounted to the wall and the glass is held onto this via collars that fit around the lamp holder. It is a straightforward light to fit.