A minefield of confusing dates and series!
The character began with reprint material from the US comics sometime in 1944 and the first issue was number 54. A few more issues followed numbered #55-58 and they all had 16 pages.
Then came a letterpress edition with no number followed by a larger format gravure edition which may have as yet unrecorded variations.
Then came the first main series in June 1950 beginning with issue #50 and it ran until issue #84 as a monthly publication. The issues variously had 28, 32 or 36 pages.
Then came the second main series numbered Vol 1 No 1 in August 1953 which ran 24 issues until Janury 1954. It was a weekly 28 page comic.
Marvelman then replaced Captain Marvel Adventures with issue #25 owing to a legal dispute between the Captain Marvel's US publisher Fawcett and the National Periodical Publications/DC, the latter accusing the former of plagiarism.
Fawcett settled out of court and the Captain Marvel title ceased publication. And so did the British reprints.
Note: some PR issues may have no value. Selected issues in FN, VF or VF/NM may not exist in some titles.