Nope, still haven't finished kicking this one around yet. The "everyone stopped brewing Stout in Britain during WW I and drank Guinness instead" theory. The one really irritatingly repeated in the introduction of the 2008 Good Beer Guide.
It's not that difficult to check. I've got figures for Guinness sales in Britain for the relevant period. Tell me if I'm making a false assumption here, but, if Guinness had taken over pretty much the whole Stout market in Britain, you'd expect their sales to have increased significantly after 1920. Wouldn't you? Let's see what actually happened.
In absolute terms, Guinness sales in Britain only increased slightly post WW I, from just over 1 million barrels a year to 1.2 - 1.3 million. In terms of the beer consumed in Britain, the percentage did indeed increase, from just under 3% to between 4% and 5%. But their market share had already been increasing pre war. In fact, between 1900 and 1912 their share of the British market doubled. As you can see below.
What happened after 1914 was just a continuation of trend of increasing sales that started well before hostilities. If anything, WW I and its aftermath put Guinness at a disadvantage in the British market. Irish independence left Guinness's Dublin brewery outside the UK and its beer liable to import duty. This was what prompted Guinness to build the Park Royal brewery in London in the 1930's.
To put Gunness's 4-5% share into perspective, between the wars 25-30% of Whitbread's output was Porter and Stout. The market for black beer was considerably larger than the output of Guinness.