There is a lot of misleading media and tweet talk about how the DexEU Secretary of State was not leading negotiations - not least from those close to the last two incumbents. It is based on a fundamental misunderstanding of how negotiations work
The lead officials supported by an extensive team are always the day to day negotiators. The many different decisions that have to be made in this process go up to the Ministers concerned. That will include the Secretary of State.
However in any significant negotiation like Brexit the PM will of course take a personal interest in the direction of negotiations. She can overrule the Secretary of State. That is how our Cabinet system works. Ministers have to live with it.
Of course there are always tensions at the top between the principals, and issues of personal trust / communications etc will arise. There will have numerous long meetings to decide strategy. In the end though the PM decided, as indeed she should. /end
