Forced retro: (disclaimer - I've no third-party confirmation of this, yet)
We're going live next week (August 2013), and we've recently discovered that forced retro is not required for active employees. This makes sense because:
a) Retro creates tables RTI and RTINI only in the current result (not in any of the retro-calculated results);
b) RTI-payroll creates table RTINI, which has only cumulative fields, so no retro can be required;
c) RTI-payroll creates table RTI, which has all cumulative fields, except for two. These are Taxable-Pay-TP and Tax-Paid-TP, where TP means "this period". Again nothing that requires retro;
d) Finally, I've done a check - create payroll results with and without forced retro, and compare tables RTI & RTINI - both produce the exact same result.
You should be able to try this out on your own system.
If you think about RTI as a monthly version of P14/P35, then it makes even more sense. All the information that HMRC needs is cumulative, and comes from cumulative results-tables CRT and CNIC.
I've asked for confirmation from colleagues, but haven't heard from anyone yet.
Forced retro for leavers: having said the above, you do have to ensure that anyone paid in this tax year is included in the first FPS (because there is no longer year-end submission to take care of this).
To get leavers into the first FPS, you have to create a payroll result with RTI-tables. To do this, you have to force-retro them into the post-RTI zone.
Regards
Mark von Delft
Edinburgh