When did these run? I don't recall seeing them at Waterloo during my commuting days. I do recall Waterloo at Paignton services; and I'm pretty sure that there was one with a single 158 that went via Salisbury and Bristol into Wales, whether it was to Cardiff or Swansea I can't recall.
From around 2003 until December 2009 when all workings west of Exeter St Davids were withdrawn to ensure there was enough stock to run the new hourly service from Exeter.
There was never a weekday service which is probably why you never saw it on the boards. There was a down train to Penzance on Saturday evenings IIRC, although it might have come from Brighton rather than London.
On Sundays there was a direct Penzance - Waterloo service setting off around early afternoon IIRC. West of Plymouth I think it was crewed by Wessex Trains / GWR staff - not sure on the precise arrangement.
I think Waterloo - Paignton services appeared in around 1994 shortly after the 159s took over - they were definitely running by 1996. Plymouth was added as a destination around 2000. For some years the majority of London - Torbay services ran from Waterloo and SWT provided Ivybridge's only London service.
158s did run from Waterloo to West Wales and up to Manchester Piccadilly via Newport and Hereford. The intention was them to provide connections into the Nightstar running from Waterloo to Paris, but this never happened.