A question for UK site workers

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So I’ve been employed by another person for 10 years and am fully trained in basically all aspect roofing. Recently left the job due to a silly discrepancy and now Im getting my CSCS card in a few weeks and looking to go and do some site roofing. I have my own tools and transport however have no idea about how it works in a sense of getting work on site, do I go site knocking and talk to the agents? Do I approach roofers on those sites? I just need some help understanding how to approach it. ...