In terms of insurance basics needed are
Public Liability - i.e. for causing damage to property or third parties (members of the public)
Employers Liability - for any employee, trainee or sub contractor (labour only). If you are a Limited company then any director would also require this (exception being if you are a sole director with no staff or employees)
Motor Fleet/Commercial Vehicle - road cover for your wagons or cars
Cost depends on the size and nature of the company. previous trading history of you and staff, locations being worked upon (houses or shops/ industrial or commercial) heights worked to, offshore works. if you are members of anyone (CITB, NASC, CISRS etc). You will need a rough idea of first years trading for Turnover and Wages (including how many guys you will employ at any given time)
Hope this helps.....