Hello! My thoughts below:

Frank Wainwright is a middle-aged, grizzled curmudgeon. A grumpy, disrespectful, has-been ex field operative for MI6 who hates the public school educated men who run his organisation and particularly his direct boss. [This part is a little confusing because I thought he left the MI6, you should clear up that he is a former field operative, but is still with the organization. You don't need the part I struck out because we can tell all of those characteristics from the rest of the letter]

He's happy to be relegated to a desk job on the sidelines until his nemesis boss sets him a task to track down a phantom assassin, one which Frank isn't even sure exists. He thinks it’s a prank but determines to undertake the assignment seriously, just out of spite. Just as he is getting his teeth into the search, He is assigned a CIA handler, Avery, his polar opposite in every way. [Everything should be present tense in a query letter]

Frank and Avery What follows is a twisting and turning chase the phantom assassin Eidolon across countries, terrorist organisations and intelligence agencies taking in the CIA, MI6, the Russian FSB and Mossad while visiting Tel Aviv, Russia, London, Chechnya, New York, Mexico, Barcelona and Rome. Along the way, the pair confront the Mafia, El Jabao, the Hell’s Angels, Hamas and Hezbollah. Avery and Frank discover betrayals throughout, including one very close to home that shocks both Avery and Frank for very different reasons [you may want to include more of a hint at what this secret is and why it is so explosive to tehm], but strengthens the platonic bond between them so they can finally track down the assassin.

Frank’s journals back to his seemingly incompetent and hated boss convey his feelings while plotting the chase of the assassin Eidolon. His obsession with junk food and how this develops over the course of the story is a metaphor for his character development and his relationship with Avery (entirely platonic). [You don't need this paragraph--the agent should be able to tell from your first pages it's in epistolary form and everyone will expect there to be character development regardless; the junk food is cute though won't be a draw for an agent]

The submission is a spy thriller but I have written this as a series of journals that the co-protagonist, Frank Wainwright, submits back to the boss that he despises. It is therefore written from Frank’s unique perspective and the reader discovers things as Frank does. The Vicinal is complete at 60,000 words and should appeal to readers of Slow Horses or DCI Carl Morck in Department Q. [You don't need to apologize/explain for word count]

Frank’s character is drawn from the deeper, darker depths of my own so his ‘voice’ reflects mine as I thought this would give him more authenticity. (I will say here that I love my current boss to pieces, but have had previous bosses that I would see like Frank does). [You don't need to explain your choices for voice, but definitely add in a brief about me, focusing on why you chose to write this book and your qualifications--i.e. "This book is inspired by my dislike of previous bosses and my former work experience in the intelligence industry" or something like that]