Could be one of a hundred things causing it. The most common things are generally a track defect or recently completed engineering works. Somebody with a copy of the weekly notices for that part of the country or access to a late notice case which covers that line would be able to give you a specific answer, but again, I would guess at a track defect or as a result of recently completed maintenance.
The "R" board is a reminder board (I think the name in the rulebook might actually be a 'repeater' but it's slipped my mind at the moment and I'll be buggered if I am going to check at this hour) and it's purpose is to remind the driver about the speed restriction ahead after having stopped at the station on which the "R" board is sited. This only happens on station platforms where the temporary speed restriction warning board is sited before the station due to the minimum distance required between the warning and commencement boards of a temporary speed restriction.