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Retention applies to both Head Contractors (both a procuring contractor and a supplying contractor) and subcontractors (generally a supplying contractor only) and is usually set at 5% of the value of the works.
This percentage is deducted from all of the interim payments made to the head contractor from its employer/client which in turn deducts it from all the subcontractors.
If the procuring contractor does not release retention when you have put pressure on them, you should take the matter extremely seriously. It may be a sign that they are experiencing financial difficulties and urgent action may be required.