Thanks to all at Greenock who enquired about my travel plans.
In the end, I took the earliest train from Greenock to Glasgow and blagged my way onto an early replacement bus to Carlisle. This was followed by a second replacement bus to Preston, where I missed my original connection by ten minutes. But I was now on a functioning rail network and able to return home on alternate trains.
So, I set off an hour earlier than planned and got home an hour later. Could have been worse.