Hi jk
This is one of those perennial questions which will sometimes get forceful arguments as organisations have made decisions, perhaps without thinking through all the pros and cons.
If we ignore all the other parallel requirements for risk assessment, Reg 3 of the Management Regs 1999 tells a primary duty holder to do a risk assessment and Reg 3(3) as amended starts:
(3) Any assessment such as is referred to in paragraph (1) or (2) shall be reviewed by the employer or [relevant self-employed person] who made it if—
(a) there is reason to suspect that it is no longer valid; or
(b) there has been a significant change in the matters to which it relates;
......with Reg 3(6) reminding employers that they DON'T have to record everything!
(6) Where the employer employs five or more employees, he shall record—
(a) the significant findings of the assessment; and
(b) any group of his employees identified by it as being especially at risk.
POWRAs (under whatever name) can be a useful tool for work where actual circumstances may be substantially different to those which were imagined when a risk assessment for any given task was first done.
Summed up by some commentators as Work as Done (WAD) v Work as Imagined (WAI).
If those doing the work are not involved in the initial risk assessment there is a good chance that there is a gulf between WAD as reflected in the "risk assessment" and WAI and hence it is definitely helpful to belatedly involve those actually at risk!
So, perhaps before asking whether that POWRA is required and deciding how formal or otherwise to make it the first question is whether the right people have been involved in putting together the "risk assessment".
Then, you need to consider how likely it is that WAI is different from WAD to the extent that you can foresee that the risks will NOT be adequately controlled at the sharp end.
Which like so many things in H&S is a judgement call.
Problem is that far too many people think that MORE paperwork = better H&S.
Partly it is often about trying to protect backsides when things go wrong and that is often NOT conducive to a pragmatic approach to MANAGING the risks.
As Kate indicates perhaps the other question is whether doing a POWRA adds value?