I have not read books 1-10 so this is a standalone.
First off, this could have been a novella if an editor had hacked out all the repetition, and that nonsense story about the duplicate key. The whole wedding thing, I suppose, is to pad out the story while showing a lot of what life was like then.
Next up, we get three suspects; one is eliminated (because he said he didn't do it and they believed him) and the second obviously did it, so the reader knows it has to be the third one.
Max seemed to be well aware that they couldn't spend the stolen money, and that they couldn't buy anything without ration books. They'd also have to have a story about why they weren't at the front. It's a stretch to think that even people this stupid would just shrug and carry out the plan anyway.
Also, they sneaked back into England and ... got to London how? slept where? ate how? Are we to believe that they slithered into a pub and immediately met the one man in all of England who was planning a bank robbery and needed exactly three men who were up for a little light larceny? And that man went to the pub expecting to see able-bodied men when we are clearly told that almost all such are at the front.
But don't get me started about the union pamphlet, possession of which seems to be about equal to mass murder, and indeed even knowing that it exists means you're probably a Bolshevik. I know it was a big issue, but this is a tad over the top.
I'm being generous with two stars. I had my suspicions when I saw up front that Marston had written 88 other books before this. He was born in 1940, so at least he's had some time to churn them out.