Can the software take 'as needed' withdrawals from pensions as tax free cash only?

This is not possible at present. As needed withdrawals (those taken automatically by the software to fulfill expenses) are assumed to be UFPLS, each being comprised of a combination of tax-free and taxable income. There is no option to set the software to take as needed withdrawals only from tax free cash. 

There is, however, an option to schedule planned withdrawals of tax free cash only from a money purchase using Flexible Access Drawdown. You will find this option on the new Planned Withdrawals screen. If you schedule planned withdrawal and set the Pensions Withdrawal Strategy to Flexible Access Drawdown, the amount set to be withdrawn (crystallised) annually will be grossed up, with the software taking the planned income only from the tax free element. This FAD option is available only for planned withdrawals, not for withdrawals taken on an as needed basis.