Accused of bullying
A. Essentially, you should respond to a solicitor just as you would to the parent, except that your language is adjusted to match the impersonal, professional approach.
You should first check that the incidents to which the solicitor is referring are the same ones that you have already investigated. If they are, and you have kept proper records of the facts, you can respond by providing that information. If the incidents are new, you will have to conduct another investigation and respond accordingly.
It is highly unlikely that the solicitor has been informed of the problems which this pupil has experienced and you might give some information about those, taking care to confine yourself to matters which can be substantiated.
If yours is an LEA school, you should let the office have a copy of the letter and your response, just in case any legal action is subsequently instigated.