The only thing that makes me think different is why would SWT▸ want to hire surplus stock when they are cutting back in other areas? Though they do seem to be the third party ROSCO» whenever a TOC▸ needs a unit or two.
There are normally 27 159s diagrammed daily from 30 units. That is pretty much normal
DMU▸ utilisation of 90% - so where's the big surplus? If
FGW▸ have two all but one will be in use that day. Perhaps they are covering with 450s elsewhere, or for the odd day they have tweaked the maintenance plan. It will be interesting to see what they do on the day.
I therefore doubt the units could be released for a long term period, and as for the units hanging around Salisbury depot all day, it isn't the case in the peaks. Like all the other SE commuter TOCs, a lot of stock is free mid day, buit it comes out again late afternoon.
As for the 158s running over the third rail all day that's a completely different question
AFAICS▸ ...
Paul