They had no right to nail or screw a fence to yours on your property. It's known as trespass in common law and they are liable if they damaged it. I would even go as far as to say that this could even be classed as criminal damage and the police could take action for what they did.

Good fences make good neighbours.

Talk to your neighbour about painting the new fence and leave it at that - don't even think about compensation unless the fence caused some real damage to something valuable like a car.
6 ft is a legitimate height for a fence - that's why so many 6ft fences are sold.

Look on this as having got a new fence for the cost of a tin of paint.

I think that the 2 metre/6ft rule for fences may not apply to front gardens.

True Lutleyyeltz! There could be something in the property deeds - especially newer properties - that restricts front fences - eg those mis-guided housing developments that ban any sort of boundary fence or wall at the front.
Local planning regulations might apply to fences over 6ft / 2m.

On your deeds it will show whether the boundary is your responsibility or theirs - it is usually one side to each neighbour. If it is their boundary to maintain, then you are stuck with it really - but I think you can ask for it to be kept to 6ft. Paint it any colour you like your side! If it is your responsibility - then you have just got them to pay for it! I would just grow something you like up it or paint it and enjoy the privacy!!