I'm obviously doing something wrong, but could somebody let me know the secret to open the boot with the foot sensor everytime?

It took me a while to understand what you needed to do, just don't wave your leg around. As that won't work. Just imagine you are simply drop kicking a ball. Good luck

Does this work, even when the car is locked?

Also what is the middle button (the one that looks like a car with the boot open) for on the key fob?

It will open the boot, (even if locked) so long as you have your keys with you. I.e. in your pocket

The middle button is usedto open the boot, even if away from the car. So long as you're within range of the cars sensor it will open. Personally, I've managed to open my boot within around 15-20m. Good luck.

Just tried the middle button and it worked from about 30cm and the side lights came on (is that because it's nearly dark?). Shut the boot, locked the car and walked away. Then walked back and tried it again and no luck. Is there a time thing going on?

Not too worried TBH, just nice to get working.