Don't think there are any, the 4x57s are used for the sleeper. But someone may have to back me up, not ventured onto the GWML,yet....
Hope this helps, Sam
Only the ECML and WCML have rescue locos strategically stabled, all other lines just rely on whatever is around.
Old Oak Common would be where 2 of the 57's are, with the other 2 down at Laira or Penzance, but this would depend on availability.
3 at Old Oak Common in the day, 1 at Penzance Long Rock. They only go to Laira if they require heavy maintenance or repairs. As people have said above, rescue locos are just what's around. Class 59s have rescued trains a couple of times as well.