Thanks HD - I've already trawled the AIP and can't find anything relevant, though that's not to say someone else might trip over something.
With regard to your point about the SLP existing to require a speed reduction before the holding area, surely the maximum holding speed requirement for the hold (220kts at BNN?) makes that speed reduction inevitable anyway.
I quite agree that a request "and what speed would you like?" tagged on to the readback of the heading change would clarify - but it is all extra chatter if the R/T's busy and your chaps in the ATC Forum seemed to think that pilots were unreasonable if they considered a heading change on a STAR to represent cancellation of the SLP Speed requirement.
I was just trying to find a definitive answer, one way or the other, to avoid misunderstandings between "us up here and you down there".
From their thread, incidentally, it would seem that the problem stems from ScACC and EDI arrivals.