In the mid-1980s the general feeling outside the Southern Region was that the 33s were over-maintained and underworked compared to other diesel classes, being diagrammed for more frequent maintenance and works visits than, say, your average 37. The SR's usual response when challenged was that they needed this level of maintenance to reduce the risk of failure on one of the main lines out of London in the peaks.
Coincidence or not, it was around this time that they started to be diagrammed for more work off the Southern; Portsmouth-Cardiff as already mentioned, but also some work in the West Country, South Wales and up the Marches to North Wales and Manchester. It was fairly easy to diagram a loco so it could go from Eastleigh onto Portsmouth-Cardiff for a day, finish at Bristol or Cardiff and then the following day pick up a Marches diagram, finish back at Cardiff again and then back to Eastleigh the following day.
Come sectorisation, most of the 33s ended up nominally allocated to freight or departmental use as the 155s and later 158s were in place by then, however the Civil Engineer's locos in particular weren't averse to turning up on passenger diagrams. In my time studying at Bristol I caught 33116 one December Saturday morning in 1990 working a Southampton stopping service with a 4TC unit vice a DEMU and a couple of weeks later the same pairing was on Waterloo-Salisbury duties (which were all theoretically Class 50s by this time). February the following year saw 33025 Sultan commandeered for an Exeter-Barnstaple diagram following overnight snow, so the traction and route knowledge was still available in 1991.