I did wonder when I saw a Southern train joined to an FCC▸ train at Wimbledon last week why neither had any logos on them. This now makes sense. Will those FCC trains in Southern colours be repainted in a TSGN scheme or eventually returned to Southern as they are?
The 319s (the older units on Thameslink) all go off lease and are replaced by new Desiros, class 700s. Some of the 319s are still in SN livery but are nothing to do with SN anymore. It would normally be these you saw at Wimbledon. Then there are 23 dual voltage Electrostars, class 377/5, these were subleased to FCC by SN from new, they are all in FCC livery but will either go back to SN routes, or possibly be used on GN route to Kings Lynn. A handful of older 377/2 are with Thameslink on short term loan from SN and are still in SN livery.
I think that's about it. The movement of stock resulting from the new Thameslink stock is anything but straightforward, the 319s could end up almost anywhere from the NW to the Thames Valley, no one really knows what the detailed plans are.
Paul