Trade Insurance is what you need for Part Time trading / repairs etc.
Tradex and Tradwise are the one the main ones but you will need to go through a broker and get a so called telephone interview / quote, hunt around as they can vary even though you'll end up with the same insurance company.
Dont mention anything do with scrap or breaking cars unless thats what you mainly do. they will expect you to turnover at least 10 cars a year, or you will not be considered a Trader. You will need to keep records of each Car you deal with and any vehicle in your hands for more than a few weeks needs to go on thier MID.
I pay around £550 full com with Tradewise, and it covers me for a couple of my own vehicles and my own Business which is separate to my part time vehicle trading up to a value of £5000 claim.
Down side is no windscreen cover which is small price to pay when your getting all your vehicles covered.
Chris.