Well soon peak rail trains will be running into the mainline station on the disused platform.
This would explain the temporary nature of Matlock riverside, and my guess is that it will either be dismantled or retained as a park and ride. As far as I'm aware progress at Matlock mainline station is... well... progressive, so the inconvenience could disappear very soon indeed.
http://www.peakrail.co.uk/Projects.html