It makes a great story, but having your heroine go on reserve duty for 6 months and come away with more knowledge than MI5 and be the only one to figure out a bombing is kind of silly. If you can get over the disbelief, the story is quirky and I liked it. 3 things annoyed me:
1. If MI5 knew Phillip had an illegitimate son, the Queen knew. They work for the Queen.
2 . The documents, an old play, you never learn if it is a forgery. Mystery/thrillers work because you try to see if you can figure them out and learn if you did. So functionally, no matter how great the idea, the story does not work.
3. A man who has not seen his 30 year old daughter since she was two, lets the first words out of his mouth be he is her father. No. Also MI5.
4. I hate stories where you introduce a clue, feature it prominently and then go nowhere with it. A bit of mystery, but the challenge coins, especially the Plan X that gives the Title, is held by both father and son yet he never knew that was the mission. The coins are never explained so unless they were GPS locators, they are just annoying and take up a good bit of the story.
Entertaining idea that could have been executed better.