We missed the open day last weekend, but yesterday, walking the dog along Lakes Road, as you do, and feeling nosy, I went down the drive to the Garden House to have a look, and I'm pleased to report that the path from there to Marple Bridge along the river bank is now open!
It's a lovely path, winding through the trees by the river, until it emerges in the car park of the Midland. If you want to do it in the opposite direction, just walk into the small car park behind the Midland (it's the upper level, on the right, not the lower one on the left), and you'll find the start of the path at the back between the trees.
I don't think it's a right of way - it's referred to as a 'permissive path' - and I suspect you are not supposed to take a bike on it.