I'm a big fan of the novel (and of Len Deighton in general), and was not disappointed by the TV adaptation. I didn't notice the sound issues many viewers have complained about (but did think it was a bit strange that Riley seemed to be doing a weird John Hurt impression while playing Archer).
SS-Standartenfuehrer Dr Huth was spot-on; he's probably the most charismatic character in the book (in spite of being a cold-hearted, amoral career Nazi), and the same holds true in the series. Liked the way his SS sidekick cracked up over the quip about needing to leave something for the jury to decide, while the man himself deadpanned it like a boss...
Regarding the PoD, I think we just have to accept that in this particular timeline, Sealion somehow succeeded. It's pretty explicit in the novel that that's what happened, even if Deighton doesn't go into any detail on exactly how. IIRC, in his non-fiction book about the Battle of Britain, "Fighter," (which I would highly recommend as a good read, even if it is somewhat dodgy as a historical source) he expresses the opinion that if the Luftwaffe could have somehow won air supremacy over SE England, the invasion plan could have succeeded, in spite of its many flaws. I don't think more recent scholarship supports this in any way, to say the least, but perhaps in the mid-late 70s less was known about the generally piss-poor state of the German plans and preparation for Sealion...?
Considering that fighting in Great Britain seems to have lasted for some months following the invasion, and to have been costly for the invaders (at least in the book, where I think the surrender document reproduced in the endpapers is dated Feb 1941), a deferred Barbarossa and hence an extended Molotov-Ribbentrop pact seems quite plausible imho.
Kellerman, so far, is exactly as portrayed in the novel, but I hope we get some indication in future episodes (as in the book) that the bumbling oaf persona is largely an act and he's just as much of a careerist Nazi shark on the quiet as Huth. Deighton seems to have been fond of this sort of character; KGB Colonel Stok who features in Funeral in Berlin and Billion Dollar Brain is pretty much just the Russian version of Kellerman.
