From the 442 thread (talking about the 350/2's):
Battery trains. Although nobody has leased them yet, there are 37 of these trains so they could replace the 442s, 158s (used with batteries) and some 159s (used with batteries).
Are you sure there are enough?
There is no way SWR will take the risk of wasting all the money they have spent on the 442s (if they still have the franchise that is?)
Plus the 442s still have further upgrades yet to go. It's pointless; passengers will complain no matter what. SWR did what they were asked to do.
By my maths you could run the all the 442 diagrams with 27/37 units (running then as 12 coach trains all day) that leaves 10 for the WofE services which didn't go to Exeter (I'm not sure that you'd want to test battery power door that far). Given that you'd need 6 to run an hourly service of one unit, 10 isn't going to get you very much capacity.
Conversely if you ran those services as all 12 coach trains you'd need 18/37 of them. You wouldn't need many to run the Salisbury 6 services (Salisbury Romsey via Southampton), by my maths is only 3, so that's a total of 21/37.
You could then look at other options, such as replacing the Basingstoke Stoppers with them so that you could increase the frequency between Basingstoke and Salisbury to 4tph (by extending the stopping services to Salisbury), that would require an extra 16 units to do so (assuming 2 units run together). That would max out the 37 units.
However with the uplift in capacity due to the more frequent service between Basingstoke and Salisbury it would be reasonable to drop a unit a Basingstoke on the Yeovil services during the off peak, which should give you enough spare units for covering for maintenance and breakdowns. Even if it didn't you could have the Basingstoke Stoppers turn back at Basingstoke every few hours.
Whilst dropping a unit at Basingstoke would take up platform capacity, this would be offset by the loss of the termination of the Basingstoke Stoppers.