By Ross Hawkins
BBC Political correspondent
Discussions between party leaders have taken place amid strict secrecy
The official was polite, but firm as he stopped me. "I'm sorry," he said. "No press allowed there."
The part of the parliamentary estate where David Cameron - and many other MPs - have their offices was unusually, and suddenly, declared out of bounds to journalists on Sunday night.
Inside the Tory strategy for negotiations with the Liberal Democrats was under discussion. It was hardly surprising someone had decided reporters were best kept away.
But it illustrated a wider point - the parties are trying very hard to ensure the fine details of these negotiations do not leak.
Conservative members of parliament were, however, welcome in the Tory leader's office on Sunday. He will also meet them on Monday, and Nick Clegg is also due to see his MPs.
The Liberal Democrat leader needs three-quarters of his parliamentary party, and of its federal executive committee, to endorse any plans he has to form an alliance.
The situation for the Conservatives is less formal, but no less important.
The easy part?
In short, Nick Clegg and David Cameron are considering going into government together because they have enough MPs between them to outvote all their rivals.
They need to be confident those MPs will back any agreement, and ensure it survives parliamentary votes.
The leaders have several means of persuading their more junior colleagues - chief among them the prospect of real power.
That may be enough, if not to convince doubters then at least to keep them silent in public.
More talks are also expected between the Conservative and Lib Dem negotiating teams.
And Gordon Brown is waiting, ready to continue his conversations with Nick Clegg should they fail.
None of the politicians want to hold their discussions in public, or communicate through the media.
But, with the challenge of negotiating a government into existence before them, keeping secrets from the press may prove to be the easy part.