As a client, do I have to provide information about the site to contractors I am thinking of appointing to carry out the work?

Yes. You need to provide those bidding for the work (or those preparing to carry out the work) with relevant information in your possession, or with information that can be obtained by sensible enquiries, including surveys and other investigations where necessary. This allows those bidding or preparing for the work to consider these hazards when making their bids or plans, and allows them to allocate resources to control the risks that will arise from these hazards. The level of detail should be proportionate to the risks involved in the project.