Thanks for your response which agrees with the Acas interpretation. I did eventually after 35 minutes get through to the Employee's SSP Help[line who gave me conflicting advice regarding PIWs and 4 days clear S/L.
The individual attends work by agreement for 4 hours per day if she feels able.
Well it seems that she is entitled to some SSP pro rata. You don't pay SSP for the first 3 days as you probably know and an employee cannot accumulate more than 28 weeks SSP. If she would normally work 8 hours a day according to her contract of employment and she works 4 then she would get half and half wages and SSP. Obviously you will have to juggle the payments to meet the realities of her attendance at the work place.
It is clear that the legislation supports phased returns to work as a means of building up worker overall health.
Please be so kind as to rate me before you leave the Just Answer site.