The whole issue is about big vs. small, i.e. Power and it's abuse..., rather than simply supermarkets using supplier credit to supplement their cash flow. However, as current supplier payment periods are 'justified' by the fact that 'other retailers do it', and suppliers in turn try to pass the cost of credit back up the supply chain via demands for equivalent credit periods from their suppliers, it is perhaps an idea to try to correct the problem by compelling retailers to pay within an appropriate period, related to rate of sale.
Given that most food retailers turn their goods over 20 times per annum on average, and credit is meant to bridge the cash gap between delivery of goods & services to a reseller, and payment by a shopper, then it could be said that payment should be made in 2.5 weeks.... i.e. a lot shorter than current levels of 40+ days in the UK.
In practical terms, products should by grouped in bands related to rate of sale: say twice weekly, weekly, 2.5 weeks, and monthly (a product moving at less than 12 times per year, perhaps should be in another channel...).
This would then allow revised terms to cascade back up the supply chain, and thus allow at least one 'concession' to be removed from the negotiation table.....