"would NATS want to maintain 2 centres and the associated doubling of running costs, given the fact Swanwick is big enough to hold both?".
Not sure I buy the "doubling of running costs" if that is simply as a result of the creation of an iScotland.
The current set up provides for 2 centres managed with a single HQ. As (essentially) a private company, I fail to see what significant additional costs NATS would incur by continuing to provide the exisiting ATS over iScotland and the NE Atlantic, under the same charging principal, whilst keeping PC open for that purpose.
If, unlikely as it is, another small/medium sized country in Europe invited NATS to manage its ATS provision would NATS say no? Or if the technology was in place say 'yes', but only if they can manage it from a single centre located in rUK?
Should an iScotland Govt. say to NATS please carry on and keep PC open, I simply can't see NATS walking away from the revenue for the sake of having to keep PC open. (I suspect the IAA would be waiting in the wings to jump in and propose a joint Irish/Scottish venture/FAB if NATS were to say no. Would NATS be keen to see that happen?).