Update: I've resolved the problem I had, tho I am still not sure it's the best (as it means I have to teach Educators on how to use it) but I have obtained approval to implement it.
For anyone else out there who has the same (probably a very basic conundrum) very simply, I've used the Calculation option on the category that contained the grade item with RPL, and then found I had to alter the Course total with a calculation (summing the category totals) as by default it seemed to sum the grade-able items.
I choose this rather than using the grade-able item's option of using a multiplicator and/or Offset as it just didn't seem to work with altering the individual items grade. Maybe I was using or viewing it incorrectly ...mmm ... the learning never stops.