Long-postponed postscript to my earlier posts on this thread, after re-reading my fading copy of "Convoy is to scatter" by Jack Broome, escort commander of PQ17.

The information available to Broome when he withdrew his escort was contained in the three admiralty signals:

"Cruiser force to withdraw to the westwards at high speed"
"Owing to threat from German surface forces convoy PQ17 is to disperse and proceed independently to Russian ports etc."
"Convoy is to scatter"

Because it wasn't directly addressed to him, Broome received the first of these only after the other two. The rising tone of panic of the second and third gave the impression that Tirpitz was only just over the horizon. Finally, due to navigational errors, the Cruiser force was in view (they should have been out of sight), and Broome could see them "treading on the gas". Unaware that they were withdrawing and under the impression that they were going to fight Tirpitz, Broome gathered his miscellany of destroyers and went with them.

Oddly, one factor which may have influenced his decision was not mentioned in his book. Before being appointed to command HMS Keppel and the First Escort Group, Broome had been Chief Staff Officer to Admiral Percy Noble, Commander of the Western Approaches Command. In this position, Broome must have been aware that the German Naval Enigma codes were being regularly cracked, and may therefore have placed more trust in the Admiralty's instructions than they warranted.

Broome's career didn't appear to suffer as a result of PQ17. He was promoted to Captain, commanded the escort carrier HMS Battler (I think - he doesn't say which) and collected a DSO. He left the Navy after the War; there were more senior officers than ships to command, and his habit of doodling acerbic caricatures of admirals made him some enemies.

Broome and Admiral Dudley Pound (the First Sea Lord who ordered PQ17 to scatter) met when HMS Keppel paid off, but neither man could apparently bring himself to broach the subject of PQ17.