Touch wood I might have discovered what is causing this and- possibly- even sorted it.
Apparently any changes in light levels can cause an optical mouse to wake the computer
. In lieu of a mouse blanket ( or a patchwork quilt -there's a project for the van) it suggests removing the check on the "Always Allow this Device to Wake the Computer" in the power management section of the Device manager (Mouse).
Also apparently activity on the network can cause it and there is a similar tag to stop that waking the computer, in the same place under Network Adapter.
Clearly this is something that Windows have met before !