I would say it depends what work you are going to be carrying out for them , window cleaning, pressure washing ,softwashing ,etc will all need a separate rams and appropriate insurance cover , any certificates of h&s qualifications like Iosh, nebosh , any training qualifications for the type of work you will be doing for them , any trade affiliations like chas or safecontracter etc will all help , or ask them what they expect you to provide to save yourself going ott if all they are after is liability insurance.
