About the only thing that would bring this show back on the rails is the re-appearance of Ben Mercer. However I fear that the original premise is long gone.
Annie's wistful looks after with her dalliances with impossibly good-looking agents from other countries in series 2 gave some hope that she had some personal integrity left.
Whatever does she think she is doing trying to play Simon Fischer, a man that she has been told is very dangerous and who she suspects is playing her? Are we to think that she does it because she enjoys the sex? It lacks credibility both that her new handler would put her in that position, and that she would go along with it. If it doesn't end in tears then it should.