Looking online it is hard to find the specific rules for Taser use by the police. I have found this from the National Police Chiefs' Concil:
Why use [Conducted Energy Devices (CED)] at all?
CEDs provide an additional option to resolve situations, including the threat of serious violence, which can come from any section of the public.
And the Police Foundation:
Currently Taser is only to be used where firearms would have been authorised or where an officer faces severe violence such that he or she needs to use force to protect the public, themselves and/or the subject
And the College of Policing:
Taser should only be used as a proportionate response to an identified threat. It should not be used to simply gain compliance with instructions or procedures where compliance is not linked to such a threat
There was a case in the news earlier this year where they seem to have been used outside this situation. It is described so, by the 80 year old target of the taser who was awaken by the officers, and was in his pajamas:
“At least four fully-dressed police officers carrying machine guns shone dazzling flashlights in my face, all shouting and shouting something unintelligible at me. They gave me no instructions, nothing.
“Suddenly one of them fired this Taser at me and the electricity started to zap. It hit me in the stomach and upper leg and I fell on the kitchen floor, powerless.
“Then they jumped on me, knelt on my back to pin me to the tiled floor, twisted my arms behind my back, handcuffed me and told me I was under arrest for a fight.”
This sounds completely contrary to the statements above. Is there any criminal law that restricts police Taser use? What charges could be brought in such situations, and what defenses could be used?