Sorry you haven't had any replies yet so I thought I'd let you know how the insurance works for me.
I have General (Public & Products Liability) Insurance with £5m cover which costs me about £75 per year. This covers my legal liability for injury and/or loss of damage to property etc occurring from my business.
In addition I have £100k worth of Professional Indemnity at a further £168 per year. This covers any possible negligence which is what I might be guilty of if I get something wrong or give questionable advice to a customer or damage any of his property.
Others testers who I have talked to don't bother with this (more expensive) aspect of cover but in my business where I often work with landlords of rented accommodation they sometimes ask for advice on their responsibilities as a "duty holder" and that is why I have decided to include the additional cover.
I also think that now the 4th Ed CoP weighs more heavily on Risk Assessment, testers are bound to become more involved with advising customers; and quite rightly too. We know what needs to be done to keep users safe and often our customers don't.
My insurance is provided by a company called Markel (UK) Ltd
Hope that helps . . . . . .