wrote

What is going to happen with the FCC Thameslink bays at Moorgate once the
last train leaves, which I understand will be on 20 March.

That's what most of this thread is about. They will remain available for
several months, but no passenger trains are likely to use them. After the
junction at Farringdon is severed, the consensus here is that there will
then be no rail use for them.

Peter