I used Trafalgar, through the IOSH Extras page in 'MyIOSH'.

OK for routine risk areas, depends if you intend to do the more hazardous end of the industry.

Other companies are available. Google produces page after page for looking at. 

Maybe find someone you know who uses this type of insurance and ask them, or a broker. It's all pretty similar on cost range for cover, at the ordinary end 2 or 3 million is a common level of cover, depending on risk type and your client requirements some companies require 5 or even 10 million in cover to get onto their books.