The only Sikorski's approach to the British Government in 1940 was the memorandum to Ernest Bevin, Britain's Minister of Labour. It didn't mention the Oder-Neisse Line, at all.
I have checked the book by Sarah Meiklejohn Terry, "Poland's Place in Europe: General Sikorski and the Origin of the Oder-Neisse Line, 1939-1943", and it shows that the above statement is factually incorrect. Sikorski's memorandum of November 1940 addressed to Bevin did propose the movement of Poland's western border to the Oder, although it did not specifically name the Oder-Neisse Line as such.
This is what she writes from page 85 onward:
Ironically it was just at this point - that is, November 1940 - that we find the first comprehensive statement of Sikorski's conception of a post-war settlement corresponding to Poland's interests and security, here in the form of a memorandum addressed to British Labour Party leader and minister of labour in the War Cabinet, Ernest Bevin. This was followed over the next two by three additional such statements, each consisting of a memorandum or set of memoranda prepared in connection with one of the general's three trips to the United States. For the first and second of these trips - in April 1941 and March 1943 - the documentation is incomplete and secondary accounts contradictory. However, for the third and most important visit, in December 1942, we have available the full texts of the several memoranda submitted as well as a variety of supporting primary and secondary materials.
While these four statements differed, often markedly, in detail, tone and emphasis - each reflecting the exigencies of the respective moments - nonetheless all were set in the conceptual framework of "real guarantees" which Sikorski had established in the closing months of 1939. All stressed the need for a compact regional organisation to enable the small nations of Central and East-Central Europe to resist German expansionism, in whatever form it might occur in the future; all projected either implicitly or explicitly a substantial shift westwards of Poland's boundary with Germany; all signalled, though in differing degrees, Poland's desire for a long-term reconciliation with the Soviet Union; and, finally, all posited the right of the Central European nations to participate in the decisions that would determine their fate.
Terry goes on to say that the Bevin memorandum contains the first mention in an official Polish document of the Oder River line. She quotes the relevant parts of the memorandum that propose the annexation by Poland of German territory.
Page 89:
....Twice - in the 18th century and presently - Germany has attempted to settle this quarrel by annexing the province of Pomerania to Germany. From the Polish point of view, the only solution that would prevent the perennial renewal of this quarrel would be the incorporation of East Prussia and Danzig into Poland, which among other things would put an end to constant German claims to the territory between the Reich proper and East Prussia......[and] would deprive Germany of the main German outpost in the east, whose only importance for the Reich is that it constitutes a springboard for expansion in the east of Europe.........In addition, Poland would not be, as in 1939, threatened by Germany on two fronts, west and north, with the northern front (East Prussia) dominating Poland's central provinces and lying scarce more than 100 km from the capital.
With regard to the question of Poland's western boundary with Germany, Terry quotes the memorandum as calling for "the strategic necessity of a shortening of that line, and "in particular of moving the German boundary away from the Polish ports of Gdynia and Gdansk as well as from the mouth of Poland's one great river, the Vistula". She then quotes the next sentence, which refers to the Oder:
Page 90:
A change of this kind would be justified by the existence on the border regions of Germany of a population either Polish by language or of Polish origin - as well as by the fact that the German territory to the east of the River Oder, in particular Prussian Pomerania- is sparsely populated.
Terry takes the view that the mentioning of the Oder immediately after the passage about moving the German border away from the mouth of the Vistula indicates that Sikorski was thinking of the Oder as the line to which that border should be moved, although he did not specifically ask for it.
Terry then quotes a passage in the memorandum which she sees as mounting a justification for a claim to all of Silesia, not just the Oppeln district which had been claimed by Poland in 1919:
In any event, from the Polish point of view, it is essential to draw attention to the fact that so-called Prussian Silesia is inhabited to a very significant degree by a population of Polish origin or language. In addition, the detachment from Germany of this region - a region that comprises one of the most important economic arsenals of German military potential - would be an effective blow to future German aspirations toward hegemony in Europe.
Although this memorandum from Sikorski to Bevin represented the first official approach by the Polish Government-in-Exile to the British Government suggesting a westward shift of the Polish-German border, it was not the first time it had intended to make such an approach. Terry describes how a secret meeting with the British and French in Angers (then the seat of the Polish Government-in-Exile was scheduled for 10 May, 1940, for which the Poles had prepared documentation making claims for major gains in the southern and northern sectors, including the Oppeln district and Eastern Pomerania up to a line running south from Kolberg, the so-called Kolobrzeg Line. However, before the Poles could present their proposal, the meeting was interrupted by news of the German offensive launched on that day.
Finally, Terry quotes the book by Leon Mitkiewicz, Sikorski's intelligence chief in the Government-in-Exile, "Z Gen. Sikorskim na obczyznie (fragmenty wspomnien", which suggests that the Poles were discussing even more far-reaching gains in the west as early as February 1940. The passage is in Mitkiewicz's diary entry for 10 February, on page 15 of the book, where he writes:
Footnote 41 on page 85 of Terry's book:
.....im Section III of the General Staff, they (Klimecki, Marecki, Noel) are already seriously pondering the problem of Poland's future boundaries and the solution of the question of East Prussia and Lower Silesia. Our western boundary has according to them several variants: the most extensive reaches to Szczecin and Frankfurt on the Oder as well as Wroclaw, while maintaining our eastern boundary.
The information from Mitkiewicz indicates that some elements in the Polish Government-in-Exile were considering a western border for Poland that essentially corresponded to the later Oder-Neisse Line, or a variant of it.
To my mind, Terry has demonstrated that the concept of pushing the German-Polish border to the west was not an initiative by Stalin, but had been adopted by Sikorski well before 1941, at a time when Stalin was still an enemy of the Poles and had had no need to do them any favours.