We highly recommend that you consider initiating a public liability insurance policy for ventilation before beginning work since failure to have a policy in place leaves you open to liability claims which could have a very large effect on your business to the extent of bankruptcy. Having a public liability insurance policy in place frees up capital which would otherwise be retained for legal expenses and such like to cover any liability claims which may be made against you. For the small cost of a public liability insurance policy it just doesn’t make sense to leave yourself without a policy and open to receiving the full costs of a liability claim.
Employers liability insurance is a legal requirement if you have any employees, regardless of how often they work for you. Failure to have a policy in place could result in a fine or even a custodial sentence. An employers liability insurance policy for ventilation would usually cover you for (among other things) employees who hold you liable for an injury which occurred while working under your direction. This is obviously one major item that an employers liability policy would usually cover you for.