Flat rate VAT and cash accounting

Have a new client who I've put on the flat rate scheme. First return due and he tells me that he has a number of invoices for the period that have not been paid. HMRC website says that cash based VAT is not allowable for those on the flat rate scheme - and that it has its own cash based turnover "method". 

The only guidance there is under the flat rate scheme is this:

"Cash-based turnover method

This method allows you to account for your VAT liability when you receive payment. It does not affect the time of supply (tax point). So if your flat rate percentage changes, you must apply the rate that was in place at the time of supply and not the rate that is in place when payment is received."

Is t really that simple - anyone have any experience of cash based VAT on the flat rate scheme?