Is This The Spy Who Didn't Come In From The Cold?

Back in March, I read "The Conscience of The King" - a novel set in Stuart times, the hero being one Henry Gresham - a spy working for Robert Cecil [Elizabeth 1's spymaster] involving a conspiracy involving William Shakespeare. [for my review, click here] I really enjoyed this. 
So I checked out the library and was able to borrow Rebel Heart - this time Gresham is working to prevent the overthrow of the Queen by the Earl of Essex. 

This was not quite as good - but I enjoyed the 'background' story, telling more about Henry and Jane, and the relationship with his servant Mannion.
I ordered the other two Greshams from the library - they arrived in fairly short order.
My third Gresham was Desperate Remedy

This one centres on the Gunpowder Plot. Yet again, Stephen takes actual historical events, real people, and established facts, but adds his own twists and somehow makes this fictional spy a central character in the action.
It took a while to get into this one - I was a little more au fait with the facts, and the names of the conspirators, and the business of the Monteagle Letter. But Gresham's role in the affair felt a little too contrived.

Finally I tackled Galleon's Grave - Gresham and the Spanish Armada. This one I found a real struggle. Admittedly I was trying to read it whilst fighting the soporific effects of my antihistamine pills, but I kept dozing off, then having to go back and re-read passages to make sure I knew where I was.
Gresham is a double agent - except at one point it felt like he was a 'spy working for England pretending to be a spy for Spain who is pretending to spy for England' [I think...but I was rather confused] Didn't enjoy it at all.
But here is a further complication - the order in which I read them, and when they were written, compared to the date in which they were set

Conscience of the King -2004 - set 1612 [James 1]
Rebel Heart - 2007 - set 1598 [Elizabeth 1, but getting old]
Desperate Remedy - 2002 - set 1605 [James 1]
Galleons Grave - 2005 - set 1588 [Elizabeth 1]

The Armada business is alluded to in the other books, yet was the last to be written. Was Stephen trying to tie up the ends, and write the backstory? and is this why it seemed so convoluted? I didn't actually warm to Gresham at all in GG. I suspect if I had read this one first, I'd not have bothered with the rest. But I am glad I did.
And I thought that was that. Until I came to write this review - when I discovered there is now a fifth book in the series! 

Having churned out four novels between 2002-2007, MS retired from being a Headteacher in 2011, and wrote The coming of the King in 2012 [set in 1603 and the accession of James 1] This one is set right in the middle of the other four. Apart from the review/sample on Amazon I cannot find the paperback on sale. Neither Dorset nor Bournemouth libraries have it in their catalogue. Amazon only have the Kindle version, and the usually reliable Abebooks don't have any copies in stock. In the 'sample' I notice there is yet another gruesome description of the beheading of Mary Queen of Scots. So I shall be giving this one a miss for the time being.
Maybe this one just didn't sell very well - perhaps this particular spy remains out in the cold?
Has anyone out there seen it/read it/got a copy ?