Hi Louise
I feel the treatment you received from your Local Trading Standards Team was completely unacceptable.
I would contact them again and ask them why they weren't intrested first time round in your concerns, and that you are now gathering evidence from a qualified electrician to show that your family were put at risk by the builder.
If they still don't seem interested in your concerns then the next course of action would be to remind them of their obligations under the Local Authority Enforcement Concordat
(Most, if not all, council departments sign up to the concordat around the country, its a semi voluntary mechanism which dictates how a local authority will be open and transparant in its approach to enforcement, and when dealing with complaints from members of the public, amongst many other things.It's basically a code of conduct)
They may say they need further evidence to investigate complaint further - which is fine, but they have to tell you what to do to get that evidence, not simply leave you hanging wondering how to go about it.
If you still get no joy tell them you would like to make an official complaint about the way you have been dealt with and also write immediately to the Cheif Executive of the council.
Wouldn't hurt for them to know you may go to the press if the electricians report proves your family were put at risk!
Also as someone also mentioned go to your local counsellor. Get them involved, tell them the trouble you faced. If the builder is as bad as you suspect then you correctly stated he may go on to cause other risk in other premises, and something definately needs to be done about that.