My apologies for my delay in responding to the various points raised, but I’ve been away for a few days.
I don’t think we should be influenced too much by the references to Groups in autobiographies. These are written by men who were very young (and probably very junior) during the war, and were thus at the end of a long line of decisions. I have little doubt that briefing officers usually/often prefaced their briefings with ‘Group says …’ or something similar, but that would have been due to the teleprinter messages containing details of an operation originating from the Group HQ. However, the Group HQs were themselves part of a chain, and their instructions/orders originated from HQ Bomber Command.
At the start of the war there is no doubt that Group HQs were responsible of operations; the number of aircraft involved was usually small (often part squadron level)and, with raids being mostly conducted in daylight the Mark 1 eyeball was as good a way of avoiding collision as any other. When daylight losses proved the folly of daylight bombing operations were switched to night but, again, numbers of aircraft involved were small, and crews usually decided their own route and time frame.
Matters only changed when aircraft from more than one Group were involved in an operation – there had to be some co-ordination otherwise the aircraft from, say, two groups would be flying on different information (and I’m not just talking about heights). When Harris arrived at HQ Bomber Command matters really did change, and although the Groups were consulted, there had to be an over-riding authority to ensure that details were consistent.
I think the clue as to what pressure setting was used can be found in Met Office history. From Dec 1940 there were daily conferences between the Met Officers at the Groups and HQ; at first these only agreed a common forecast for the night’s weather, but the forecast winds remained the responsibility of the Group Met Officers. These were found to differ enormously between Groups, and in Feb 1943 a new system was introduced whereby upper air charts were drawn to a datum of 1013.2 mb, instead of sea level, and winds calculated from the patterns produced. Only the Met Office HQ had all the necessary information, and it was responsible for the forecasts. It was, in effect, the start of pressure pattern flying.
I suspect that for Bomber Command operations the 1013.2 mb pressure setting dates from this time, but flights below the transition level continued to use QFE.
That’s as I read it from research I’ve been doing on the met office at HQ Bomber Command (hence my lack of references) but I have asked the RAF Museum for its advice on the matter.
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