My current Council does and this was an area of great dispute between the LA and our Auditors.
Northgate will have no issues paying calendar monthly payments from April 06. (Err let me change that to shouldl). The need for testing is therefore minimal as it does work (again… disclaimers / riders everything else applies in relation to any bug not yet spotted). Make sure you set your payment date to 28th for payment in arrears cases and 16th for old scheme advance cases.
Payments prior to April 06 are complicated as payments are made relating to the payment period for the month in question. This seriously complicates the calculation of under / overpayments caused by changes of circumstances. A payment for February will cover a period of four weeks, a payment for March will cover a period of 31 days. Therefore the weekly eligible rent will change over the period in question.
In deciding whether to pay calendar monthly you (as an Authority) are required to take into account the pay frequency of the claimant but administrative convenience is also a factor. The issue with regard to complicated change of circs calculations is no longer there so that cannot be used in any defence. It is easy to do, so why not just do it?