I propose to introduce a system of paying tax on profits after realisation, as is already possible with VAT.
 
So if you have not been paid by the customer, you just don't put the invoice into revenue. Similarly, you should not expense an invoice that you have not paid.
This would go some way to helping entrepreneurs who are in difficulty because of non-payment, but on the other hand many people would at least pay their invoices to their suppliers at the end of the year, if not earlier, so that they can claim the costs.