What exactly is Croder's status in TSS? As nearly as I can determine, he is not yet Chief of Signals, a position which apparently is second only to Bureau One (Shepley) by the time of QSol (because Q, to get Thrower ["tosser"?] fired, has to appeal first to his control Shatner, then to Croder, and finally to Shepley).
According to Quiller, Croder in TSS is chief of base directorate, implying that he supervises directors or at least the Bureau staff that DiFs utilize for their missions. On the other hand, Q implies that Croder himself either is or has been a director in the field (checking for tags on Croder in Tempelhof, Q notes, "as a rule the London directors aren't too good in the field" [02:33]). Moreover, Croder acts very much like a director in the field when he goes out to meet Q after Ignatov tries to kill him: he briefs people, assembles a working cell, speaks authoritatively to locals like Zoya, and supervises Bracken. At the same time, Bracken is clearly supposed to be the DiF, and, as I've noted previously, I find it curious that Croder's taking over a mission in which he's more at fault for Q's failure to kill Schrenk than Bracken is.
It's all very confusing. For one thing, though we're clearly meant to assume that "Chief of Signals" is more powerful than the "Chief of Base Directorate," somehow the latter sounds much more authoritative. For another thing, the idea of an officer over-ranking the DiF is definitely a new wrinkle in Bureau hierarchy. Here's the breakdown on DiF positions in the Q-orpus thus far:
1. TB/QM: executive (Pol) acting as contact and de facto
supervisor for another executive (Quiller).
2. T9D: DiF Loman fully in charge in field.
3. TSP: DiF Ferris fully in charge in field.
4. TWD: No DiF, control Egerton in London.
5. TTB: DiF Loman fully in charge in field.
6. TMC: DiF Ferris fully in charge in field.
7. TKM: Several DiFs in charge in various fields.
8. TSE: DiF Ferris fully in charge in field, but doubling as courier.
9. TSS: DiF Bracken starts fully in charge, but is replaced by Chief of Base Directorate.
At first, I thought that perhaps the more serious consequences expected after the shift the mission takes once Schrenk reveals (to some degree) what he intends to do, might justify Croder's coming out to take things over. But then, looking back over the missions prior to TSS, I realized that several of them (T9D, TSP, TTB, TMC, and certainly TKM) have consequences at least as important as those in TSS, and also that only in TKM is the field authority of the DiF diluted (and really, not even then, since Ferris is fully in charge once Q's locked onto the Kobra cell, even though he can't come up with appropriate directives in some of the final phases).
I can see two possibilities that might explain this apparent change in policy. The first would be a reorganization of the Bureau itself, with the chain of command altered to fit a changing geopolitical climate in the world. The second would be simply that Ferris and Loman are held to be more trustworthy than Bracken. Whatever the explanation, I think it's clear that by the time of TSS (and perhaps a little before), Adam Hall was beginning to rethink that most enduring, and paradoxically most changeable, feature of Quiller novels, the Bureau.