In most Contracts the Monthly Payment paid by the Authority is based in whole, or in part, on estimated tonnage and performance figures. This is to allow the invoice to be issued reasonably promptly at the end of each month even if some data has not been finalised. Where estimates are used there will be a reconciliation process and a further payment or refund to ensure the total amount paid by the Authority is in line with the final out-turn data.
The WIDP Contract Payment Mechanism (Schedule 4) refers to the WIDP Payment Mechanism Guidance37 on this point. This sets out an approach to dealing with reconciliations. However in practice contractual arrangements vary significantly on this issue. The Contract Management Manual should provide a reference to the relevant sections of the Contract and set out the procedures for dealing with the process.
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37 This guidance is available online at http://www.defra.gov.uk/environment/waste/local-authorities/widp/widp-procurement-pack/