Then I needed to connect the old Toshiba laptop for a while. However while it can connect to the internet via the router, can see the other two PC's and use their resources, it can no longer be seen by the rest of the network. 'Search for Computer' on either of the other PC's does not find it. Its name and workgroup (Mshome) haven't changed.
Since I've changed nothing on the Toshiba since it was last successfully networked, have I overlooked something obvious?