To add my two-pen'th - I've had a response from Stockport's Conservation Officer, Paul Hartley. The gist of this is:
He was not aware of this - and as a result of Dave's enquiry the Public Rights of Way Officer has been today to have a look. He has advised that the stone has been placed on private land, it is not clear for what purpose at this time, but there is no current scheme to repair the Roman Bridge - or build a new one from these materials.
The Rights of Way Officer is organising the use of crushed stone to repair pathways in the area but the stone which has just appeared is not for this purpose.
It appears there is nothing to be concerned about. If I learn anymore I will let you know.