I'm told by Buckingham Palace officials that whatever the hesitation may have been on the Sinn Fein side, the Queen was more than willing for this meeting to go ahead.
"Always open to it," were the words that were used.
Once, this trip was planned as part of the Diamond Jubilee celebrations. It should be stressed that Sinn Fein are saying that in their view the meeting is not part of those jubilee celebrations. They agonised about it and they voted on it. It was a majority, but not a unanimous vote.
The meeting will be held behind closed doors. It won't be a public event, but it is a historic meeting and it will be photographed for posterity.